GIFT   OF 
MICHAEL  REE.SE 


LIGHT,  VISIBLE  AND  INVISIBLE 


LIGHT 

VISIBLE    AND    INVISIBLE 

A  SERIES  OF  LECTURES 

DELIVERED  AT  THE   ROYAL   INSTITUTION   OF 
GREAT  BRITAIN,   AT  CHRISTMAS,    1896 


BY 
SILVANUS  P.  THOMPSON, 

D.Sc.,  F.R.S.,  M.R.I. 

PRINCIPAL   OF,    AND    PROFESSOR   OF    PHYSICS    IN,    THE   CITY   AND   GUILDS 
TECHNICAL   COLLEGE,    FINSBURY,    LONDON 


Neto  ¥orfc 
THE    MACMILLAN     COMPANY 

LONDON:  MACMILLAN  AND  CO.,  LIMITED 
1897 


COI'YRIGHT,  1897,  BY 

THE  MACMILLAN  COMPANY 


INTRODUCTION 

Two  things  are  expected  of  a  lecturer  who  undertakes 
a  course  of  Christmas  lectures  at  the  Royal  Institution. 
In  the  first  place  his  discourses  must  be  illustrated  to 
the  utmost  extent  by  experiments.  In  the  second, 
however  simple  the  language  in  which 'scientific  facts 
and  principles  are  described,  every  discourse  must 
sound  at  least  some  note  of  modernity,  must  reflect 
some  wave  of  recent  progress  in  science. 

So  in  undertaking  a  course  of  lectures  in  Optics  the 
lecturer  in  the  present  instance  ventured  to  proceed  on 
certain  lines  which  may,  perhaps,  seem  strange  to  the 
sedate  student  whose  knowledge  of  optics  has  been 
acquired  on  the  narrower  basis  of  the  orthodox  text- 
book. The  ideas  developed  in  the  first  lecture  arose 
from  the  conviction  that  the  time-honoured  method  of 
teaching  geometrical  optics  —  a  method  in  which  the 
wave -nature  of  light  is  steadily  ignored  —  is  funda- 

39631 


VI  LIGHT 

mentally  wrong.  For  the  sake  of  students  and  teachers 
of  optics  he  has  added  to  Lecture  I.  an  Appendix,  in 
which  the  newer  ideas  are  further  developed.  Other 
Appendices  have  been  added  to  the  later  Lectures, 
with  the  aim  of  filling  up  some  of  the  gaps  left  in  the 
subjects  as  treated  in  the  lecture  theatre. 

Now  that  the  electromagnetic  nature  of  all  light- 
waves has  been  fully  demonstrated,  no  apology  is  needed 
for  bringing  into  the  fifth  Lecture  a  few  of  the  experi- 
mental points  upon  which  that  demonstration  rests. 
That  these  fundamental  points  can  be  given  without 
any  great  complication  of  either  thought  or  language 
is  in  itself  the  strongest  argument  for  making  that 
demonstration  an  essential  feature  at  an  early  stage  in 
the  teaching  of  the  science. 

Many  of  the  ideas  which  must  be  grasped,  for 
example  that  of  the  polarisation  of  light,  are  popularly 
supposed  to  be  extremely  difficult ;  whereas  the  difficulty 
lies  not  in  the  ideas  themselves  so  much  as  in  the 
language  in  which  they  are  generally  set  forth.  In  an 
experience  lasting  over  a  good  many  years,  the  author 
has  found  that  the  main  points  in  the  phenomena  of 
polarisation  are  quite  easily  grasped  by  persons  of 
ordinary  intelligence — even  by  children — provided  they 
are  presented  in  a  modern  way  devoid  of  pedantic 


INTRODUCTION  Vli 

terms,  and  illustrated  by  appropriate  models.  A 
similar  remark  would  equally  apply  to  other  parts  of 
optics,  such  as  interference  and  diffraction,  which  are 
barely  alluded  to  in  the  present  lectures.  Many 
branches  are  necessarily  omitted  altogether  from  so 
brief  a  course :  amongst  them  the  entire  subject  of 
spectrum  analysis,  the  construction  and  theory  of  optical 
instruments,  and  the  greater  part  of  the  subject  of 
colour  vision.  No  attempt  was  made  to  include  these 
topics,  and  no  apology  is  needed  for  their  omission. 
Whatever  value  these  discourses  may  possess  must 
depend  upon  the  things  they  include,  not  upon  those 
which  they  do  not. 

LONDON,  January  1897. 


CONTENTS 

LECTURE   I 

LIGHT  AND   SHADOWS 

How  light-waves  travel — Experiments  with  the  ripple-tank — How 
shadows  are  cast — How  to  make  light -waves  converge  and 
diverge — Measurement  of  brightness  of  lights — Reflexion  of 
light  by  mirrors — Formation  of  images — Regular  and  irregular 
reflexion — Diffuse  reflexion  by  paper  and  rough  surfaces — 
Multiple  images — Refraction  of  light— Lenses — The  eye  as  an 
optical  instrument — Some  curious  optical  experiments — Inver- 
sion of  Images — The  magic  mirror  of  Japan — English  magic 
mirrors Page  I 

APPENDIX — The  general  method  of  geometrical  optics  .         .       55 


LECTURE   II 

THE  VISIBLE  SPECTRUM   AND  THE   EYE 

Colour  and  wave-length— Rainbow  tints — The  spectrum  of  visible 
colours — Spectrum  made  by  prism — Spectrum  made  by  grat- 
ing— Composition  of  white  light  —  Experiments  on  mixing 


X  LIGHT 

colours — Analysis  of  colours — Blue  and  yellow  mixed  make 
white,  not  green — Complementary  tints — Contrast  tints  pro- 
duced by  fatigue  of  eye — Other  effects  of  persistence  of  vision 
— Zoetrope — Animatograph  .....  Page  71 

APPENDIX — Anomalous  refraction  and  dispersion  .         .         .     100 


LECTURE  III 

POLARISATION   OF   LIGHT 

Meaning  of  polarisation — How  to  polarise  waves  of  light — Illus- 
trative models — Polarisers  made  of  glass,  of  calc-spar,  and  of 
slices  of  tourmaline — How  any  polariser  will  cut  off  polarised 
•  light — Properties  of  crystals — Use  of  polarised  light  to  detect 
false  gems — Rubies,  sapphires,  and  amethysts — Polarisation  by 
double-refraction — Curious  coloured  effects,  in  polarised  light, 
produced  by  colourless  slices  of  thin  crystals  when  placed 
between  polariser  and  analyser  —  Further  study  of  comple- 
mentary and  supplementary  tints — Exhibition  of  slides  by 
polarised  light — Effects  produced  on  glass  by  compression,  and 
by  heating 105 

APPENDIX— The  elastic-solid  theory  of  light  .         .         .         .156 


LECTURE  IV 

THE   INVISIBLE   SPECTRUM   (ULTRA-VIOLET  PART) 

The  spectrum  stretches  invisibly  in  both  directions  beyond  the 
visible  part — Below  the  red  end  are  the  invisible  longer  waves 
that  will  warm  bodies  instead  of  illuminating  them— These  are 
called  the  calorific  or  infra-red  waves.  Beyond  the  violet  end 
of  the  visible  spectrum  are  the  invisible  shorter  waves  that 


CONTENTS  xi 

produce  chemical  effects — These  are  called  actinic  or  ultra- 
violet waves — How  to  sift  out  the  invisible  ultra-violet  light  from 
the  visible  light — How  to  make  the  invisible  ultra-violet  light 
visible — Use  of  fluorescent  screens — Reflexion,  refraction,  and 
polarisation  of  the  invisible  ultra-violet  light — Luminescence  : 
the  temporary  kind  called  Fluorescence,  and  the  persistent 
kind  called  Phosphorescence  —  How  to  make  "luminous 
paint " — Experiments  with  phosphorescent  bodies — Other  pro- 
perties of  invisible  ultra-violet  light — Its  power  to  diselectrify 
electrified  bodies — Photographic  action  of  visible  and  of  in- 
visible light — The  photography  of  colours — Lippmann's  dis* 
covery  of  true  colour-photography — The  reproduction  of  the 
colours  of  natural  objects  by  trichroic  photography — Ives's 
photochromoscope . Page  160 

APPENDIX — Table  of  wave-lengths  and  frequencies         .         .     190 


LECTURE  V 

THE   INVISIBLE   SPECTRUM   (iNFRA-RED   PART) 

How  to  sift  out  the  invisible  infra-red  light  from  the  visible  light — 
Experiments  on  the  absorption  and  transmission  of  invisible 
infra-red  light — It  is  cut  off  by  transparent  glass,  but  trans- 
mitted by  opaque  ebonite — Use  of  radiometer — Use  of  thermo- 
pile and  bolometer— "Heat-indicating"  paint — Experiments 
on  the  reflexion,  refraction,  and  polarisation  of  invisible  infra- 
red light — Discovery  by  Hertz  of  propagation  of  electric  waves 
— Hertzian  waves  are  really  gigantic  waves  of  invisible  light 
—Experiments  on  the  properties  of  Hertzian  waves ;  their 
reflexion,  refraction,  and  polarisation — Inference  that  all  light- 
waves, visible  and  invisible,  are  really  electric  waves  of  different 
sizes 192 

APPENDIX — The  electromagnetic  theory  of  light    .         .         .     230 


xn  LIGHT 

LECTURE  VI 

RONTGEN   LIGHT 

Rontgen's  Discovery — Production  of  light  in  vacuum  tubes  by 
electric  discharges — Exhaustion  of  air  from  a  tube — Geissler- 
tube  phenomena — The  mercurial  pump — Crookes's-tube  pheno- 
mena— Properties  of  Kathode  light — Crookes's  shadows — De- 
flection of  Kathode  light  by  a  magnet  —  Luminescent  and 
mechanical  effects — Lenard's  researches  on  Kathode  rays  in  air 
— Rontgen's  researches — The  discovery  of  X-rays  by  the  lumin- 
escent effect — Shadows  on  the  luminescent  screen— Transpar- 
ency of  aluminium— Opacity  of  heavy  metals — Transparency  of 
flesh  and  leather — Opacity  of  bones — Absence  of  reflexion,  re- 
fraction, and  polarisation— Diselectrifying  effects  of  Rontgen 
rays — Improvements  in  Rontgen  tubes — Speculations  on  the 
nature  of  Rontgen  light— Seeing  the  invisible  .  .  Page  238 

APPENDIX— Other  kinds  of  invisible  light      ....     277 
INDEX 285 


LECTURE  I 

LIGHTS    AND    SHADOWS 

How  light-waves  travel — Experiments  with  the  ripple-tank — How 
shadows  are  cast  —  How  to  make  light -waves  converge  and 
diverge  —  Measurement  of  brightness  of  lights  —  Reflexion  of 
light  by  mirrors — Formation  of  images — Regular  and  irregular 
reflexion — Diffuse  reflexion  by  paper  and  rough  surfaces  — 
Multiple  images — Refraction  of  light — Lenses — The  eye  as  an 
optical  instrument — Some  curious  optical  experiments — Inver- 
sion of  images — The  magic  mirror  of  Japan — English  magic 


LIGHT,  as  is  known  both  from  astronomical  observations 
and  from  experiments  made  with  optical  apparatus,  travels 
at  a  speed  far  exceeding  that  of  the  swiftest  motion  of  any 
material  thing.  Try  to  think  of  the  swiftest  thing  on  the 
face  of  the  earth.  An  express  train  at  full  speed,  per- 
haps, occurs  to  you.  How  far  will  it  go  while  you 
count  up  to  ten  ?  Counting  distinctly  I  take  just  over 
5^  seconds.  In  that  time  an  express  train  would  have 
travelled  500  feet !  Yet  a  rifle-bullet  would  have  gone 
farther.  There  is  something  that  goes  quicker  than  any 
actual  moving  thing.  A  sound  travels  faster.  In  the 
same  time  a  sound  would  travel  a  mile.  Do  you  say 
that  a  sound  is  only  a  movement  in  the  air,  a  mere  aerial 

B 


2  LIGHT  LECT. 

wave?  That  is  quite  true.  Sound  consists  of  waves, 
or  rather  of  successions  of  waves  in  the  air.  None  of 
you  who  may  have  listened  to  the  delightful  lectures  of 
Professor  M'Kendrick  in  this  theatre  last  Christmas  will 
have  forgotten  that ;  or  how  he  used  the  phonograph  to 
record  the  actual  mechanical  movements  impressed  by 
those  air-waves  as  they  beat  against  the  sensitive  surface 
of  the  tympanum. 

But  this  Christmas  we  have  to  deal  with  waves  of  a 
different  kind — waves  of  light  instead  of  waves  of  sound 
— and  though  we  are  still  dealing  with  waves,  yet  they 
are  waves  of  quite  a  different  sort,  as  we  shall  see. 

In  the  first  place,  they  travel  very  much  faster  than 
waves  of  sound  in  the  air.  During  that  5^  seconds, 
while  an  express  train  could  go  500  feet,  or  while  a 
sound  would  travel  a  mile,  light  would  travel  a  million 
miles  !  A  million  miles  !  How  shall  I  get  you  to  think 
of  that  distance  ?  An  express  train  going  60  miles  an 
hour  would  take  i6,666f  hours,  which  is  the  same  thing 
as  694  days  10  hours  40  minutes.  Suppose  you  were 
now — 29th  December  1896,  3  o'clock — to  jump  into 
an  express  train,  and  that  it  went  on  and  on,  not  only 
all  day  and  all  night,  but  all  through  next  year,  day  after 
day,  and  all  through  the  year  after  next,  month  after 
month,  until  November,  and  that  it  did  not  stop  till 
24th  November  1898  at  20  minutes  before  2  o'clock 
in  the  morning ;  by  that  time — nearly  two  years — you 
would  have  travelled  just  a  million  miles.  But  the 
space  that  an  express  train  takes  a  year  and  eleven 
months  to  travel,  light  travels  in  5^  seconds— just  while 
you  count  ten  ! 


i  LIGHTS  AND  SHADOWS  3 

And  not  only  are  the  waves  of  light  different  from 
those  of  sound  in  their  speed — they  are  different  in  size. 
As  compared  with  sound-waves  they  are  very  minute 
.ripples.  The  invisible  waves  of  sound  are  of  various 
sizes,  their  lengths  differing  with  the  pitch  of  the  sound. 
The  middle  c  of  the  pianoforte  has  a  wave-length  of 
about  4  feet  3  inches,  while  the  shrill  notes  that  you  can 
sing  may  be  only  a  few  inches  long.  A  shrill  whistle 
makes  invisible  ripples  about  half  an  inch  long  in  the  air. 
But  the  waves  of  light  are  far  smaller.  The  very  largest 
waves  of  all  amongst  the  different  kinds  of  visible  light 
— the  red  waves — are  so  small  that  you  could  pack 
39,000  of  them  side  by  side  in  the  breadth  of  one  inch  ! 
And  the  waves  of  other  colours  are  all  smaller.  How 
am  I  to  make  you  grasp  the  smallness  of  these  wavelets  ? 
What  is  the  shortest  thing  you  can  think  of?  The  thick- 
ness of  a  pin  ?  Well,  if  a  pin  is  only  a  hundredth 
part  of  an  inch  thick  it  is  still  390  times  as  broad  as  a 
ripple  of  red  light.  The  thickness  of  a  human  hair? 
Well,  if  a  hair  is  only  a  thousandth  part  of  an  inch  thick 
it  is  still  39  times  as  big  as  the  size  of  a  wave  of  red 
light. 

Now,  from  the  facts  that  waves  of  light  travel  so  fast, 
and  are  so  very  minute,  there  follow  some  very  important 
consequences.  One  consequence  is  that  the  to-and-fro 
motions  of  these  little  ripples  are  so  excessively  rapid — 
millions  of  millions  of  times  in  a  second — that  there  is 
no  possible  way  of  measuring  their  frequency :  we  can 
only  calculate  it.  Another  consequence  is  that  it  is 
very  difficult  to  demonstrate  that  they  really  are  waves. 
While  a  third  consequence  of  their  being  so  small  is 


4  LIGHT  LECT. 

that,  unlike  big  waves,  they  don't  spread  much  round 
the  edges  of  obstacles. 

You  have  doubtless  all  often  watched  the  waves  on 
the  sea,  and  the  ripples  on  a  pond,  and  know  how  when 
the  waves  or  the  ripples  in  their  travelling  strike  against 
an  obstacle,  such  as  a  rock  or  a  post,  they  are  parted  by 
it,  pass  by  it,  and  run  round  to  meet  behind  it.  But 
when  waves  of  light  meet  an  obstacle  of  any  ordinary 
size  they  don't  run  round  and  meet  on  the  other  side  of 
it — on  the  contrary,  the  obstacle  casts  a  shadow  behind 
it.  If  the  waves  of  light  crept  round  into  the  space 
behind  the  obstacle,  that  space  would  not  be  a  dark 
shadow. 

Well,  but  that  is  a  question  after  all  of  the  relative 
sizes  of  the  obstacle  and  of  the  waves.  Sea  waves  may 
meet  behind  a  rock  or  a  post,  because  the  rock  or  the 
post  may  not  be  much  larger  than  the  wave-length.1 
But  if  you  think  of  a  big  stone  breakwater — much  bigger 
in  its  length  than  the  wave-length  of  the  waves, — you 
know  that  there  may  be  quite  still  water  behind  it ;  in 
that  sense  it  casts  a  shadow.  So  again  with  sound-waves; 
ordinary  objects  are  not  infinitely  bigger  than  the  size  of 
ordinary  sound-waves.  The  consequence  is  that  the 
sound-waves  in  passing  them  will  spread  into  the  space 
behind  the  obstacle.  Sounds  don't  usually  cast  sharp 
acoustic  shadows.  If  a  band  of  musicians  is  playing  in 
front  of  a  house,  you  don't  find,  if  you  go  round  to  the 

1  Note  that  the  scientific  term  "wave-length  "  means  the  length 
from  the  crest  of  one  wave  to  the  crest  of  the  next.  This,  on  the 
sea,  may  be  50  feet  or  more.  In  the  case  of  ripples  on  a  pond,  it 
may  be  but  an  inch  or  two.  Many  people  would  call  it  the  breadth 
of  the  waves  rather  than  the  length. 


i  LIGHTS  AND 

back  of  the  house,  that  all  sound  is  cut  off.  The  sounds 
spread  round  into  the  space  behind.  But  if  you  notice 
carefully  you  will  observe  that  while  the  house  does  not 
cut  off  the  big  waves  of  the  drum  or  the  trombone,  it 
does  perceptibly  cut  off  the  smaller  waves  of  the  flute  or 
the  piccolo.  And  Lord  Rayleigh  has  often  shown  in 
this  theatre  how  the  still  smaller  sound-waves  of  ex- 
cessively shrill  whistles  spread  still  less  into  the  space 
behind  obstacles.  You  get  sharp  shadows  when  the  waves 
are  very  small  compared  with  the  size  of  the  obstacle. 

Perhaps  you  will  then  tell  me  that  if  this  argument 
is  correct,  you  ought  not,  even  with  light-waves,  to  get 
sharp  shadows  if  you  use  as  obstacles  very  narrow 
obstacles,  such  as  needles  or  hairs.  Well,  though  per- 
haps you  never  heard  it,  that  is  exactly  what  is  found  to 
be  the  case.  The  shadow  of  a  needle  or  a  hair,  when 
light  from  a  single  point  or  a  single  narrow  slit  is  allowed 
to  fall  upon  it,  is  found  not  to  be  a  hard  black  shadow. 
On  the  contrary,  the  edges  of  the  shadow  are  found  to 
be  curiously  fringed,  and  there  is  light  right  in  the  very 
middle  of  the  shadow  caused  by  the  waves  passing  by 
it,  spreading  into  the  space  behind  and  meeting  there. 

However,  all  this  is  introductory  to  the  subject  of 
shadows  in  general.  If  we  don't  take  special  precau- 
tions, or  use  very  minute  objects  to  cast  shadows,  we 
shall  not  observe  any  of  these  curious  effects.  The 
ordinary  shadows  cast  by  a  bright  light  proceeding 
from  any  luminous  point  are  sharp-edged ;  in  fact,  the 
waves,  in  ordinary  cases,  act  as  though  they  did  not 
spread  into  the  shadows,  but  travelled  simply  in  straight 
lines. 


6  LIGHT  LECT. 

Let  me  try  to  illustrate  the  general  principle  of  the_ 


FIG. 


travelling  of  ripples  by  use  of  a  shallow  tank l  of  water, 

1  Ripple-tanks  for  illustrating  the  propagation  of  waves  have  long 
been  known.  Small  tanks  were  used  at  various  times  by  Professor 
Tyndall.  See  also  Professor  Poynting,  F.R.S.,  in  Nature^ 
May  1884,  p.  119. 


I  LIGHTS  AND  SHADOWS  7 

on  the  surface  of  which  I  can  produce  ripples  at  will. 
An  electric  lamp  placed  underneath  it  throws  up  shadows 
of  the  ripples  upon  a  slanting  translucent  screen,  and 
you  can  see  for  yourselves  how  the  ripples  spread  from 
the  centre  of  disturbance  in  concentric  circles,  each  circle 
enlarging,  and  the  ripples  following  one  after  another  at 
regular  distances  apart.  That  distance  is  what  we  call 
the  "wave-length." 

If  I  use  the  tip  of  my  finger  to  produce  a  disturbance, 
the  ripples  travel  outward  in  all  directions  at  an  equal 
speed.  Each  wave-front  is  therefore  a  circle.  If,  however, 
I  use  to  produce  the  disturbance  a  straight  wooden  ruler, 
it  will  set  up  straight  wavelets  that  follow  one  another  in 
parallel  ranks.  These  we  may  describe  as  plane  waves, 
as  distinguished  from  curved  ones.  Notice  how  they 
march  forward,  each  keeping  its  distance  from  that  in 
front  of  it. 

Now,  if  you  have  ever  watched  with  care  the  ripples 
on  a  pond,  you  will  know  that  though  the  ripples  march 
forward,  the  water  of  which  these  ripples  are  composed 
does  not — it  merely  rises  up  and  down  as  each  ripple 
comes  by.  The  proof  is  simple.  Throw  in  a  bit  of 
cork  as  a  float.  If  the  water  were  to  flow  along,  it  would 
take  the  cork  with  it.  But  no ;  see  how  the  cork  rides 
the  waves.  It  is  the  motion  only  that  travels  forward 
across  the  surface — the  water  simply  swings  to- and - 
fro,  or  rather  up  and  down,  in  its  place.  Now  that 
this  has  once  been  brought  to  your  attention,  you 
will  be  able  to  distinguish  between  the  two  kinds  of 
movement — the  apparent  motion  of  the  waves  as  they 
travel  along  the  surface,  and  the  actual  motion  of  the 


8 


LIGHT 


LECT. 


particles  in  the  waves,  which  is  always  of  an  oscillatory 
kind. 

Here  is  a  model  of  a  wave-motion  that  will  make  the 
difference  still  clearer.     At  the  top  a  row  of  little  white 


FIG. 


balls  (Fig.  2)  is  arranged  upon  stems  to  which,  in 
regular  order  one  after  the  other,  is  given  an  oscillatory 
motion  up  and  down.  Not  one  of  these  white  particles 
travels  along.  Each  simply  oscillates  in  its  own  place. 


I  LIGHTS  AND  SHADOWS  9 

Yet  the  effect  is  that  of  a  travelling  wave,  or  rather  set 
of  waves.  The  direction  in  which  the  wave  travels  is 
transverse  to  the  displacements  of  the  particles.  The 
length  from  crest  to  crest  of  the  waves  is  about  4  inches. 
Their  velocity  of  travelling  depends,  of  course,  on  the 
speed  with  which  I  turn  the  handle  of  the  apparatus. 
The  amplitude  of  the  displacement  of  each  of  the  balls 
is  not  more  than  one  inch  up  or  down  from  the  centre 
line. 

Perhaps  now  you  will  be  able  to  think  of  the  little 
wavelets  of  light,  marching  in  ranks  so  close  that  there 
are  40,000  or  50,000  of  them  to  the  inch,  and  having  a 
velocity  of  propagation  of  185,000  miles  a  second. 

Now  let  me  state  to  you  two  important  principles  of 
wave-motion — all-important  in  the  right  understanding 
of  the  behaviour  of  waves  of  light. 

(1)  The  first  is  that  waves  always1    march  at   right 
angles    to    their   own    front.     This   is    how    a   rank  of 
soldiers  march — straight  forward  in  a  direction  square 
to  the  line  into  which   they  have  dressed.     It  was  so 
with  the  water-ripples  that  you  have  already  seen. 

(2)  The  second  principle  is  that  every  point  of  any 
wave-front  may  be  regarded  as  a  new  source  or  centre 
from  which  waves  will  start  forward  in   circles.     Look 
at  the  sketch  (Fig.  3).     From  P   as   a  centre   ripples 
arc  travelling  outward  in  circles,  for  there  has  been  a 
disturbance  at  P.     Now  if  there  is  placed  in  the  way  of 

1  Always,  that  is  to  say,  in  free  media,  in  gases,  liquids,  and 
non-crystalline  solids.  In  crystals,  where  the  structure  is  such  that 
the  elasticity  differs  in  different  directions,  it  is  possible  to  have 
waves  marching  obliquely  to  their  own  front. 


10 


LIGHT 


LECT. 


these  ripples  a  screen,  S,  or  obstacle,  with  a  hole  in  it, 
all  the  wave-fronts  that  come  that,  way  will  be  stoppe'd 
or  reflected  back,  except  that  bit  of  each  wa^ve-front 
that  comes  to  the  gap  in  the  screen.  That  particular 
bit  will  go  on  into  the  space  beyond,  but  will  spread  at 
equal  speed  in  all  directions,  giving  rise  to  a  new  but 
fainter  set  of  ripples  which  will  be  again  of  circular  form, 


\  \ 

\  \ 

\  \ 

\ 

^  \ 

\          \ 
\          i 


\ 


s 


FIG.  3. 


having  their  centre  however  not  at  P  but  at  the  gap  in 
the  screen.  This  too  I  can  readily  illustrate  to  you  in 
my  ripple-tank. 

The  first  of  these  two  principles  is  really  a  conse- 
quence of  the  second,  and  of  another  principle  (that 
of  "interference")  which  concerns  the  overlapping  of 
waves.  Of  these  we  may  now  avail  ourselves  to  find 
how  waves  will  march  if  we  know  at  any  moment  the 


LIGHTS  AND  SHADOWS 


ii 


FIG.  4. 


shape  of  the  wave -from.  Suppose  (Fig.  4)  we  knew 
that  at  a  certain  moment  the  wave-front  of  a  set  of 
ripples  had  got  as  far 
as  the  curved  line  FF, 
and  that  we  wanted  to 
know  where  it  would 
be  an  instant  later.  If 
we  know  how  fast  the 
wave  travels  we  can 
think  of  the  time  taken 
to  travel  some  short 
space  such  as  half  an 
inch.  Take  then  a  pair 
of  compasses  and  open 
them  out  to  half  an 
inch.  Then  put  the 
point  of  the  compasses  at  some  part — say  a — of  the  curve 
FF,  and  strike  out  the  piece  of  circle  as  shown  at  a'. 
That  is  where  the  disturbance  would  spread  to  in  that 
short  interval  of  time  if  the  bit  of  wave-front  at  a  had 
alone  been  allowed  to  spread  forward.  But  the  bit  at  b 
is  also  spreading,  so  we  must  strike  another  arc,  using  b 
as  centre,  and  another  at  c,  and  another  at  dt  and  so  on, 
using  the  same  radius  for  all  of  them.  And  now  we  see 
that  if  all  these  bits,  instead  of  acting  each  separately, 
are  acting  at  the  same  time,'  the  wavelets  from  each  will 
overlap  and  give  us  one  large  enveloping  curve  at  GG ; 
the  effect  being  the  same  as  though  the  wave-front  FF 
had  itself  marched  forward  to  GG.  Those  parts  of  the 
wavelets  that  tend  to  spread  cross-ways  in  the  over- 
lapping balance  one  another;  for  instance,  part  of  the 


12  LIGHT  LECT. 

wavelet  from  a  tends  to  cross  downwards  in  front  of  c, 
while  a  part  of  the  wavelet  from  e  tends  to  cross  upwards 
to  an  equal  amount.  These  sideway  effects  cancel  one 
another,  with  the  result  that  the  effect  is  the  same  on 
the  whole  as  though  the  bit  of  wave  at  c  had  simply 
marched  straight  forward  to  c . 

Perhaps  you  will  say  that  if  this  is  true  then  when 
light-waves  meet  an  obstacle  some  light  ought  to  spread 
round  into  the  shadow  at  the  edges.  And  so  it  does  as 
has  already  been  said.  But,  owing  to  the  exceeding 
smallness  of  the  light-waves  compared  with  the  dimen- 
sions of  ordinary  objects,  the  spreading  is  so  slight  as  to 
be  unnoticed.  In  fact,  except  when  we  are  dealing 
with  the  shadows  of  very  thin  objects,  like  hairs  and 
pins,  or  with  mere  edges,  the  light  behaves  as  though  it 
simply  travelled  in  straight  lines.1 

Our  next  business  is  to  show  how  ripples  can  be 
made  to  diverge  and  converge.  If  we  take  a  point  as 
our  source  of  the  ripples,  then  they  will  of  themselves 
spread  or  diverge  from  that  point  in  all  directions  in 
circles,  each'  portion  of  each  wave-front  having  a  bulg- 
ing form.  If  we  take  as  the  source  a  flat  surface,  so  as 
to  get  plane  waves,  they  march  forward  as  plane  waves 

1  This  is  all  that  is  meant  by  the  old  statement  that  light  travels 
in  "rays."  There  really  are  no  rays.  The  harder  one  tries  to 
isolate  a  "ray"  by  itself,  by  letting  light  go  first  through  a  narrow 
slit  or  pinhole,  and  then  passing  it  through  a  second  slit  or  pinhole, 
the  more  do  we  find  it  impossible  ;  for  then  we  notice  the  tend- 
encies to  spread  more  than  ever.  If  the  word  "ray"  is  to  be 
retained  at  all  in  the  science  of  optics,  it  must  be  understood  to 
mean  nothing  more  than  the  geometrical  line  along  which  a  piece 
of  the  wave-front  marches. 


LIGHTS  AND  SHADOWS 


FIG.  5. 


without  either  diverging  or  converging.  If,  however,  we 
can  in  any  way  so  manage  our  experiments  as  to  get 
ripples  with  a  hollow  front 
instead  of  a  bulging  front, 
then  the  succeeding  ripples 
will  converge  as  they  march. 
This  is  shown  in  Fig.  5. 
Suppose  FF  is  a  hollow  wave- 
front  marching  forward  to- 
ward the  right.  Think  of  the 
bit  of  wave-front  at  a.  After 
a  short  interval  of  time  it 
would  spread  (were  it  alone) 
to  a.  Similarly  b  would  spread 
to  b'y  and  so  on,  so  that  when 
all  these  separate  wavelets  overlap,  the  effect  is  the  same 
as  though  there  the  wave-front  FF  had  marched  to  GG, 
closing  in  as  it  marches.  After  the  lapse  of  another 
equally  short  interval  it  will  have  closed  in  to  HH.  It  is 
clear  that,  on  the  principle  that  waves  always  march  at 
right  angles  to  their  own  front,  they  tend  all  to  march 
inwards  and  meet  at  a  new  centre  somewhere  at  Q. 
Suppose  you  ranged  a  row  of  soldiers  in  a  curve  like 
FF,  and  told  each  soldier  to  march  straight  forward  be- 
tween his  comrades.  If  each  soldier  were  to  march  at 
right  angles  to  the  curved  line,  they  would  all  be  march- 
ing toward  a  common  centre,  and  would  close  in  against 
one  another ! 

Now  it  is  obviously  easy  to  make  waves  of  light 
diverge — they  do  so  of  themselves  if  the  source  of  light 
be  a  point.  We  shall  see  later  how  to  make  them  con- 


14  LIGHT  LECT. 

verge ;  but,  meantime,  we  will  use  what  we  know  about 
divergence  to  help  us  to  measure  the  relative  brightness 
of  two  lights. 

Here  is  a  little  electric  glow-lamp.  The  shopman 
who  sold  it  to  me  says  that  when  it  is  supplied  with 
electric  current  at  the  proper  pressure,1  it  will  give  as 
much  light  as  sixteen  candles.  I  switch  on  the  current 
and  it  shines.  I  light  a  standard  candle,2  so  that  you 
can  compare  the  brightness  for  yourselves.  Do  you 
think  that  the  glow-lamp  is  really  sixteen  times  as  bright 
as  the  candle?  Your  eye  is  really  a  very  unreliable 
judge 3  of  the  relative  brightness.  We  must,  therefore, 
find  some  way  of  balancing  the  brighter  and  the  less 
bright  lights  against  one  another.  The  instrument  for 
doing  this  is  called  a  photometer, 

1  Electric  pressure,  or  "voltage,"  is  measured  in  terms  of  the 
unit  of  electric  pressure  called  the  "volt."     The  usual  electric  pres- 
sure of  the  conductors  which  branch  from  the  supply-mains  into  a 
house  is  100  volts. 

2  The   standard   candle  prescribed   by  the  regulations   of    the 
Board  of  Trade  as  the  legal  standard  of  light  in  Great  Britain  is  a 
sperm  candle  burning  120  grains  of  spermaceti  per  hour. 

3  This   unreliability  of  the  eye  to  form  a  numerically  correct 
judgment  is  partly  dependent  on  the  physiological  fact  that  the  sen- 
sation is  never  numerically  proportional  to  the  stimulus.     Though 
the  stimulus  be  16  times  as  great,  the  sensation  perceived  by  the 
brain  is  not  16  times  as  great.     The  rule  (Fechner's  law)  is  that 
the   sensation    is   proportional   to    the   natural    logarithm    of    the 
stimulus.     The  natural  logarithm  of  16  is  277  ;  that  is  to  say,  the 
light  that  is  1 6  times  as  bright  as  I  candle  only  produces  a  sensa- 
tion 277  times  as  great.     A  single  light  of  100  candle  brilliancy 
only  produces  a  sensation  4*6  times  as  great  as  that  of  I  candle. 
Besides  this  the  iris  diaphragm  of  the  eye  automatically  reduces  the 
size  of  the  pupil  when  a  brighter  light  shines  into  the  eye,  making 
the  eye  less  sensitive. 


I  LIGHTS  AND  SHADOWS  15 

But  before  we  can  understand  the  photometer  we 
must  first  think  about  the  degree  of  illumination  which 
a  light  produces  when  it  falls  upon  a  white  surface.  I 
take  here  a  piece  of  white  cardboard  one  inch  square. 
If  I  hold  it  close  to  my  candle  it  catches  a  great 
deal  of  the  light,  and  is  brightly  illuminated.  If  I 
hold  it  farther  away  it  is  less  brightly  illuminated. 
We  can,  therefore,  alter  the  illumination  of  the  sur- 
face by  altering  the  distance.  But  we  cannot  use  this 
principle  for  calculations  about  brightness  until  we 
know  the  rule  that  connects  the  distance  with  the 
degree  of  illumination ;  and  that  rule  depends  upon  the 
way  in  which  light  spreads  when  it  starts  from  a  point. 


FIG.  6. 


Suppose  we  think  of  the  whole  quantity  of  light  that  is 
spreading  all  round  from  a  point.  Of  all  that  amount 
of  light  what  fraction  will  be  caught  by  this  square  inch 
of  cardboard  when  I  hold  it  a  foot  away?  Not  very 
much.  But  now  think  of  that  same  amount  of  light  as 
as  it  goes  on  spreading.  Fig.  6  shows  you  that  by  the 
time  that  the  light  has  travelled  out  from  the  centre  to 
double  the  distance  it  will  have  spread  (according  to  the 
law  of  rectilinear  propagation  discussed  above)  so  that 
the  diverging  beam  is  now  twice  as  broad  each  way. 
It  will  now  cover  a  cardboard  square  that  is  2  inches 
eacri  way,  or  that  has  4  square  inches  of  surface.  So  if 
the  same  amount  of  light  that  formerly  fell  on  i  square 
inch  is  now  spread  over  4  square  inches  of  surface,  it 


16  LIGHT  LECT. 

follows  that  each  of  those  4  square  inches  is  only 
illuminated  one  quarter  as  brightly  as  before.  If  you 
had  a  bit  of  butter  to  spread  upon  a  piece  of  bread — 
and  then  you  were  told  that  you  must  spread  the  same 
piece  of  butter  over  a  piece  of  bread  of  four  times  the 
surface,  you  know  that  the  layer  of  butter  would  be 
only  the  quarter  as  thick !  And  so  again,  if  I  let  the 
light  spread  still  farther,  by  the  time  it  has  gone  three 
times  as  far  it  will  have  spread  over  nine  times  the 
surface,  and  the  degree  of  illumination  on  any  one  square 
inch  at  that  treble  distance  will  be  only  one-ninth  part 
as  great  as  at  first.  This  is  the  so-called  law  of  "  inverse 
squares,"  and  is  simply  the  geometrical  consequence1 
of  the  circumstance  that  the  light  is  spreading  from  a 
point.  Now  we  are  ready  to  deal  with  the  balancing  of 
two  lights.  By  letting  two  lights  shine  on  a  piece  of  card- 
board, or  rather  on  two  neighbouring  pieces,  and  then 
altering  the  distance  of  one  of  the  lights  until  both 
pieces  of  card  are  equally  illuminated,  we  can  get  a 
balance  of  effects,  and  then  calculate  from  the  squares 
of  the  distances  how  bright  the  lights  were.  The  eye, 
which  is  a  very  bad  judge  of  relative  unequal  bright- 
nesses is  really  a  very  fair  judge  (and  by  practice  can  be 
trained  to  be  a  very  accurate  judge)  of  the  equality  of 
illumination  of  two  neighbouring  patches.  But  we  must 
make  our  arrangements  so  that  only  one  light  shines 

1  The  fact  that  a  candle  flame  is  not  a  mere  point  introduces  a 
measurable  error  in  photometry.  It  cannot  be  too  clearly  under- 
stood that  the  law  of  inverse  -  squares  is  never  applicable  strictly 
except  to  effects  spreading  from  points.  This  criticism  applies  also 
to  the  use  or  misuse  of  the  law  of  inverse-squares  in  magnetism  and 
electricity. 


LIGHTS  AND  SHADOWS 


upon  each  patch.  One  simple  way  of  doing  this  is  to 
let  each  light  cast  a 
shadow  of  a  stick  on 
a  white  surface,  so  that 
each  light  shines  into 
the  shadow  cast  by 
the  other.  If  you  alter 
the  distances  till  the 
shadows  are  equally 
dark,  then  you  know 
that  the  illumination 
of  each  is  equal.  But 
a  better  way  is  to 
arrange  matters  that 
the  two  illuminated 
patches  are  actually 
superposed.  Here  is 
a  very  simple  and 
effective  way  of  doing 
it.  Two  pieces  of 
white  cardboard,  A 
and  B  (Fig.  7),  form- 
ing a  V-shape,  are  set 
upon  a  stand,  between 
the  two  lights  that  are 
to  be  compared.  One 
light  shines  upon  the 
surface  of  A,  and  the 
other  upon  the  surface 
of  B.  Through  A  are 
cut  a  number  of  slots  or  holes,  so  that  the  illuminated 

c 


i8  LIGHT  LECT. 

surface  of  B  is  seen  through  the  slots  in  A.  If  the 
illumination  of  A  is  duller  than  that  of  B  the  slots  will 
seem  dark  between  the  brighter  bars  of  the  front  card ; 
but  if  the  illumination  of  A  is  brighter  than  that  of 
then  the  slots  will  seem  bright  between  dull  bars.1  By 
moving  one  of  the  lights  nearer  or  farther  away,  the 
respective  illuminations  can  be  altered  until  balance  is 
obtained;  and  then  the  relative  values  are  calculated 
from  the  squares  of  the  distances.  With  this  photometer 
let  us  now  test  our  electric  lamp  to  see  if  it  is  really 
worth  sixteen  candles.  I  put  it  on  the  photometer 
bench  and  move  it  backward  and  forward  till  the  lights 
balance.  You  see  it  balances  when  rather  less  than 
four  times  as  far  away  as  the  standard  candle.  It  is, 
therefore,  of  not  quite  sixteen  candle-power. 

Another  very  simple  and  accurate  photometer  is 
made  by  taking  two  small  slabs  of  paraffin  wax  (such  as 
candles  are  made  of)  and  putting  them  back  to  back 

1  This  form  of  photometer  is  a  modification  by  Mr.  A.  P. 
Trotter,  M.A. ,  of  Cape  Town,  of  the  relief  photometer  invented  in 
1883  by  the  author  and  Mr.  "C.  C.  Starling.  To  prevent  error 
arising  from  internal  reflexion  the  back  of  the  card  A  should  be 
blackened.  By  setting  the  support  at  a  fixed  distance  from  the 
standard  light  on  the  left  side,  and  altering,  as  needed  to  obtain 
balance,  the  distance  of  the  light  of  which  the  brightness  is  to  be 
measured,  it  is  possible  to  make  the  instrument  direct-reading ;  the 
scale  to  the  right  of  the  support  being  graduated  so  as  to  read  not 
the  actual  distances  but  their  squares.  For  instance,  if  the  distance 
of  the  middle  slot  from  the  standard  light  be  I  metre,  then  on  the 
other  side  the  graduation  must  read  I  at  I  metre  ;  4  at  2  metres ; 
9  at  3  metres,  and  so  forth.  Accuracy  of  reading  is  promoted  by 
the  circumstance  that  when  balance  has  been  found  for  the  middle 
slot  of  A  the  slots  to  the  left  of  the  middle  will  look  darker,  and 
those  to  the  right  brighter  than  the  central  one. 


i  LIGHTS  AND  SHADOWS  19 

with  a  sheet  of  tin-foil  or  black  paper  between  them. 
They  are  then  placed  (as  in  Fig.  8)  on  the  graduated 
bench  between  the  lights  whose 
brightness  is  to  be  compared  to- 
gether, and  set  in  such  a  way  that 
one  light  shines  on  one  paraffin 
slab,  and  the  other  light  on  the 
other  slab,  as  in  Fig.  9.  If  the 
illuminations  on  the  two  sides  FlG-  9- 

balance  the  edgjs  of  the  slabs  will  seem  equally  bright. 
But  if  the  illumination  on  one  face  is  stronger  than 
that  on  the  other  then  that  paraffin  slab  which  is 
more  highly  illuminated  will  seem  brighter  at  its  edge 
than  the  other.1  This  is  because  of  the  translucent  or 

1  This  paraffin  slab  photometer  is  the   invention  of  Dr.   Joly, 
F.R.S.,  of  Dublin.     It  is  an  exceedingly  satisfactory  instrument. 


FIG.  10. 


Either  of  these  two  forms  of  instrument  here  described'is  preferable 
to  the  old-fashioned  "grease-spot"  photometer  of  Bunsen.  But 
both  are  surpassed  in  accuracy  by  the  precision  -  photometer  of 


20  LIGHT  LECT. 

semi-opaque  property  of  paraffin  wax,  which  results  in 
a  diffusion  of  the  light  laterally.  With  this  photometer 
it  is  very  easy  to  balance  the  brightness  of  two  lights, 
even  if  their  tint  be  not  quite  identical.  In  Germany, 
they  employ  as  standard,  instead  of  a  sperm  candle,  the 
little  Hefner  lamp  filled  with  a  chemical  liquid  known 
as  amyl-acetate.  But  it  has — as  you  see — the  serious 
disadvantage  of  giving  out  a  light  which  is  unfortunately 
of  a  redder  tint  than  most  of  our  other  lights.  To  be 
quite  suitable,  the  lamp  that  we  choose  as  a  standard  of 
light  ought  to  be  not  only  one  that  will  give  out  a  fixed 
quantity  of  light,  but  one  that  is  irreproachable  in  the 
quality  of  its  whiteness :  it  should  be  a  standard  of 
white  light.  Perhaps  now  that  acetylene  gas  is  so 
easily  made  it  may  serve  as  a  standard,  for  as  yet 
none  of  the  proposed  electric  standards  seem  quite 
satisfactory. 

Let  us  pass  on  to  the  operation  of  reflecting  light  by 
means  of  mirrors.     A  piece  of  polished  metal  such  as 

Brodhun  and  Lummer,  which  can,  however,  only  be  described  here 
very  briefly.  It  gives  determinations  that  can  be  relied  on  to  within 
one-half  of  one  per  cent.  The  two  lights  to  be  compared  are  caused 
to  shine  on  the  two  opposite  faces  of  a  small  opaque  white  screen,  W 
(Fig.  10).  The  eye  views  these  two  sides,  as  reflected  in  two  small 
mirrors,  Ml  and  M2,  by  means  of  a  special  prisrn-combination,  con- 
sisting, as  shown,  of  two  right-angled  prisms  of  glass,  A  and  B, 
which  are  cemented  together  with  balsam  over  only  a  small  part  of 
their  hypotenuse  surfaces ;  the  light  from  Ml  can  pass  direct  through 
this  central  portion  to  the  eye,  but  the  uncemented  portions  of  the 
hypotenuse  surface  of  B  act  by  total  internal  reflexion  and  bring  the 
light  from  M2  to  the  eye.  The  eye,  therefore,  virtually  sees  a  patch 
of  one  surface  of  W  surrounded  by  a  patch  of  the  other  surface  of 
W,  and  hence  can  judge  very  accurately  as  to  whether  they  are 
equally  illuminated  or  not. 


LIGHTS  AND  SHADOWS 


21 


silver,  or  a  silvered  glass,  will  reflect  the  waves  of  light, 
and  so,  though  in  an  inferior  degree,  will  any  other 
material  if  only  its  surface  be  sufficiently  smooth.  By 
sufficiently  smooth  I  mean  that  the  ridges  or  scratches 
or  roughnesses  of  its  surface  are  decidedly  smaller  than 
the  wave-length  of  the  light.  If  the  scratches  or  ridges 
on  a  surface  are  in  width  less  than  a  quarter  of  the  wave- 
length (in  the  case  of  light,  therefore,  less  than  about 
inch)  they  do  not  cause  any  breaking  up  of 

8 

Q 


the  waves ;  and  such  surfaces  are,  for  optical  purposes, 
quite  "smooth."  Indeed  that  is  the  usual  way  of 
polishing  things.  You  scratch  them  all  over  with  some 
sort  of  very  fine  powder  that  makes  scratches  finer  than 

awcou  of  an  incn- 

Now  the  rebound  of  waves  when  they  beat  against  a 
polished  surface,  whether  that  surface  be  a  flat  one  or 
a  curved  one,  can  be  studied  by  applying  the  same 
principles  of  wave-motion  that  we  have  already  learned. 
In  Fig.  ii  we  have  light  starting  from  a  point  at  P  and 


22  LIGHT  LECT. 

spreading.  If  a  smooth  obstacle,  SS,  is  placed  in  the 
path  of  these  waves  they  will  meet  it,  but  some  parts  of 
the  wave-front  will  meet  it  before  other  parts.  Think 
of  the  bit  of  the  wave-front  that  meets  the  mirror  at  a. 
If  it  had  not  been  stopped,  it  would  after  a  brief 
moment  of  time  have  got  as  far  as  a.  But  having 
bounded  back  from  the  surface  it  will  set  up  a  wavelet 
that  will  spread  backwards  at  the  same  rate.  Therefore, 
draw  with  your  compasses  the  wavelet  a",  using  as  radius 
the  length  a  a.  The  next  bit  of  the  wave -front  b 
reaches  the  surface  of  the  mirror  a  little  later.  The 
length  from  thence  to  b'  is  therefore  a  little  shorter  than 
a  a.  So  take  that  shorter  length  as  radius  and  strike 
out  the  wavelet  b" .  Completing  the  set  of  wavelets  in 
the  same  way  we  get  the  final  curve  of  the  reflected 
wave,  which  you  see  will  now  march  backwards  as 
though  it  had  come  from  some  point  Q  on  the  other 
side  of  the  mirror.  In  fact,  if  the  mirror  is  a  flat  one, 
Q  will  be  exactly  as  far  behind  the  surface  as  P  is  in 
front  of  it.  We  call  the  point  Q  the  "  image  "  of  the 
point  P.  This  reflexion  of  ripples  as  though  they  had 
come  from  a  point  behind  the  mirror  I  can  show  you 
by  aid  of  my  ripple-tank.  I  put  in  a  flat  strip  of  lead  to 
serve  as  a  reflector — see  how  the  waves  as  they  come  up 
to  it  march  off  with  their  curvature  reversed,  as  though 
they  had  started  from  some  point  behind  the  reflecting 
surface. 

Again  I  can  show  you  the  same  thing  with  a  candle 
and  a  looking-glass.  You  know  that  we  can  test  the 
direction  in  which  light  is  coming  by  looking  at  the 
direction  in  which  a  shadow  is  cast  by  it.  If  I  set  up 


i  LIGHTS  AND  SHADOWS  23 

(Fig.  12)  this  little  dagger  on  a  whitened  board  I  can 
see  which  way  its  shadow  falls.  If  now  I  place  a  candle 
beside  it  on  the  board  at  P  it  casts  a  shadow  of  the 
dagger  on  the  side  away  from  P.  Next,  set  up  a  piece 
of  silvered  mirror  glass  a  little  farther  along  the  board. 
We  have  now  two  shadows.  One  is  the  direct  shadow 
which  was  previously  cast ;  the  other  is  the  shadow  cast 
by  the  waves  that  have  been  reflected  in  the  mirror,  and 


you  see  by  the  direction  in  which  this  second  shadow 
falls  that  it  falls  just  as  if  the  light  had  come  from  a 
second  candle  placed  at  Q,  just  as  far  behind  the  mirror 
--'as  P  is  in  front/  Let  us  put  an  actual  second  candle  at 
Q,  and  then  -take  away  the  mirror,  and  you  see  the 
second  shadow  in  the  same  place  and  of  the  same  shape 
as  before.  So  we  have  proved  by  direct  experiment  that 
our  reasoning  about  the  waves  was  correct.  Indeed, 


24  LIGHT  LECT. 

you  have  only  to  look  into  a  flat  mirror,  and  examine 
the  images  of  things  in  it,  to  satisfy  yourselves  about 
the  rule.  The  images  of  objects  are  always  exactly 
opposite  the  objects,  and  are  each  as  far  behind  the 
mirror  as  the  object  is  in  front.  Probably  you  have 
all  heard  of  the  savage  prince  captured  by  sailors, 
who,  when  he  was  taken  on  board  ship  and  shown  a 
mirror  hanging  on  a  wall,  wanted  to  run  round  to 
see  the  other  savage  prince  whom  he  saw  on  the  other 
side! 

If  instead  of  using  flat  mirrors  we  use  curved  ones, 
we  find  different  rules  to  be  observed.  That  is  because 
the  curved  surfaces  print  new  curvatures  on  the  wave- 
fronts,  causing  them  to  alter  their  lines  of  march.  There 
are,  as  you  know,  two  sorts  of  curvatures.  The  surface 
may  bulge  out — in  which  case  we  call  it  convex ;  or  it 
may  be  hollowed — in  which  case  we  call  it  a  concave 
surface. 

In  my  ripple  tank  I  now  place  a  curved  piece  of 
metal  with  its  bulging  side  toward  the  place  where  I 
make  the  ripples.  Suppose  now  I  send  a  lot  of  plane 
ripples  to  beat  against  this  surface ;  the  part  of  the 
wave-front  that  strikes  first  against  the  bulging  curve  is 
the  earliest- to  be  reflected  back.  The  other  parts  strike 
the  surface  later,  and  when  reflected  back  have  fallen 
behind ;  so  that  the  ripples  come  back  curved — the 
curved  mirror  has,  in  fact,  imprinted  upon  the  ripples 
a  curvature  twice  as  great  as  its  own  curvature.  This 
can  be  seen  from  Fig.  13,  where  we  consider  the  straight 
ripples  marching  to  meet  the  bulging  reflector.  The 
middle  point  M  of  the  bulging  surface  meets  the  advan- 


LIGHTS  AND  S 


cing  wave  first  and  turns  that  bit  back.  If  there  had 
been  no  obstacle  the  wave  would,  after  a  short  interval 
of  time,  have  got  as  far  as  A.  But  where  will  it  actually 
go  to  ?  The  bit  that  strikes  M  will  go  back  as  far  as 
B ;  the  bit  marked  a  will  go  on  a  little,  and  then  be 
reflected  back.  Take 
your  compasses  again 
and  measure  the  dis- 
tance it  still  has  to  go 
to  a,  and  then  turn- 
ing the  compasses 
strike  out  the  arc  a. 
Do  the  same  for  the 
bits  marked  b  and  ^, 
and  you  will  find  the 
overlapping  wavelets 

to  give  you  the  new  outline  of  the  reflected  wave, 
which  marches  backwards  as  though  it  had  started  from 
the  point  marked  F.  This  point  F  is  half-way  between 
M  and  the  centre  of  curvature  of  the  surface.  The 
centre  is  marked  C  in  the  drawing. 

So,  again,  if  I  use  as  reflector  a  hollow  or  concave- 
curved  surface,  it  will  imprint  upon  the  waves  a  concave 
form,  the  imprinted  curvature  being  twice  as  great  as 
the  curvature  of  the  reflecting  surface.  But  now  we 
come  upon  a  new  effect.  See  in  my  ripple-tank  how, 
when  the  straight  ripples  beat  against  the  concave 
surface,  so  that  the  middle  part  of  the  wave-front  is  the 
last  to  rebound,  all  the  other  parts  have  already  re- 
bounded and  are  marching  back,  the  returning  ripples 
being  curved  inwards.  In  fact,  you  see  that,  being 


LIGHT 


LECT. 


themselves  now  curved  ripples  with  hollow  wave-fronts, 
they  converge  inwards  upon  one  another,  and  march  back 
toward  the  point  F.  A  bit  of  the  wave -front  at  P 
marches  straight  until  it  strikes  the  mirror  at  R.  Then 
instead  of  going  on  to  Q  it  is  reflected  inward  and 
travels  to  F,  toward  which  point  other  parts  of  the  wave 
also  travel.  Here  then  we  have  found  a  real  focus  or 
meeting  point  of  the  waves;  not,  as  in  the  preceding 
cases,  a  virtual  focus  from  which  the  waves  seemed  to 


FIG.  14. 

come.  We  have  then  learned  that,  for  ripples  at  least, 
a  concave  mirror  may  produce  a  real  convergence  to  a 
point. 

Let  us  at  once  show  that  the  same  thing  can  be  done 
with  light-waves  by  using  a  concave  silvered  mirror. 

From  my  optical  lantern,  with  its  internal  electric 
lamp,  my  assistant  causes  a  broad  beam  of  light  to 
stream  forth.  The  air  is  dusty,  and  each  little  particle 
of  dust  catches  a  portion  of  the  beam,  and  helps  you 
to  see  which  way  it  is  marching.  In  this  beam  I  hold  a 


LIGHTS  AND  SHADOWS 


27 


concave  silvered  mirror.  At  once  you  see  how  by  print- 
ing a  curvature  upon  the  waves  it  forces  the  beam  to 
converge  (Fig.  15)  upon  a  point  here  in  mid-air.  That 
point  is  the  focus.  You  will  further  notice  that  by 
turning  the  mirror  about  I  can  shift  the  position  of  the 


FIG.  15. 

focus,  and  concentrate  the  waves  in  different  places  at 
will. 

If  I  replace  the  concave  mirror  by  a  convex  one,  I 
shall  cause  a  divergence  of  the  waves.  No  longer  is 
there  any  real  focus,  but  the  waves  now  march  away 
as  if  they  had  come  from  a  virtual  focus  behind  the 
mirror  at  F  (Fig.  16),  precisely  as  we  saw  for  the  ripples 
in  the  ripple-tank. 

We  have  now  got  as  far  as  the  making  of  real  images 


LIGHT 


LECT. 


by  so  changing  the  shapes  of  the  wave-fronts  and  their 
consequent  lines  of  march  as  to  cause  them  to  converge  to 
focal  points.  Let  us  try  a  few  more  experiments  on  the 
formation  of  images.  Removing  from  the  optical  lantern 
all  its  lenses,  let  us  simply  leave  inside  it  the  electric 
lamp.  You  know  that  in  this  lamp  there  are  two  pencils 
of  carbon,  the  tips  of  which  do  not  quite  touch,  and 


FIG.  16. 

which  are  made  white-hot  by  the  flow  of  the  electric 
current  between  them.  I  cover  up  the  opening  in  front 
of  the  lantern  by  a  piece  of  tin-foil,  and  in  this  I  now 
stab  a  small  round  hole  with  a  pointed  stiletto.  At 
once  you  see  thrown  on  the  screen  an  image  (Fig.  17) 
of  the  two  white-hot  tips  of  the  carbon  pencils.  The 
positive  carbon  has  a  flat  end,  the  negative  tip  is  pointed. 
That  image  is  inverted  as  a  matter  of  fact,  and  its  forma- 
tion on  the  screen  is  a  mere  consequence  of  the  rectilinear 


LIGHTS  AND  SHADOWS  ' 


29 


propagation  of  the  light.  If  I  stab  another  hole  we  shall 
have  another  image.  This  time  I  have  pierced  a  square 
hole,  but  the  second  image  is  just  the  same  as  the  first, 
and  does  not  depend  on  the  shape  of  the  hole.  I  pierce 
again  a  three-cornered  hole — still  another  image.  If  I 
pierce  a  whole  lot  of  holes  I  get  just  as  many  images, 
and  they  are  arranged  in  a  sort  of  pattern,  which  exactly 
corresponds  to  the  pattern  of  holes  I  have  pierced  in 
the  tin-foil. 

Now  if  I  wanted  to  produce  one  single  bright  image 
instead  of  a  lot  of  little  images  scattered  about,  I  must  in 


FIG. 


some  way  manage  so  to  turn  these  various  beams  that 
they  shall  all  converge  upon  the  same  region  of  the  screen. 
In  other  words,  the  formation  of  bright  images  can  be 
effected  by  using  some  appliance  which  will  imprint  a 
convergence  upon  the  waves.  You  know  that  a  concave 
mirror  will  do  this.  Very  well,  let  me  use  a  concave 
mirror.  See  how,  when  we  choose  one  of  the  proper 
curvature  to  converge  the  light  upon  the  screen,  it  blends 
all  the  images  together,  and  gives  us  one  bright  image. 
We  may  remove  our  tin-foil  cap  altogether,  so  as  to  work 


30  LIGHT  LECT. 

with  the  whole  beam,  and  we  get  a  still  more  brilliant 
image  of  the  carbon  points. 

Substituting  for  the  arc-lamp  a  group  of  little  coloured 
electric  glow-lamps,  I  cause  their  beams  to  be  reflected 
out  into  the  room  by  my  concave  mirror,  and  here,  by 
trying  with  a  hand -screen  of  thin  translucent  paper, 
you  see  how  I  can  find  the  real  image  of  the  group  of 
lamps.  This  image  is  inverted ;  and  being  in  this  case 
formed  at  a  distance  from  the  mirror  greater  than  that 
of  the  object,  it  is  magnified.  If  the  object  is  removed 
to  a  greater  distance  the  image  comes  still  nearer  in ; 
and  is  then  of  diminished  size,  though  still  inverted. 

So  far  we  have  been  dealing  with  the  regular  reflexion 
that  takes  place  at  properly  polished  surfaces.  But  if 
the  surfaces  are  not  properly  polished — that  is,  if  their 
ridges  or  scratches  or  roughnesses  are  not  sensibly  smaller 
than  the  size  of  waves,  then,  though  they  may  still 
reflect,  the  reflexion  is  irregular.  White  paper  reflects 
in  this  diffuse  way.  You  do  not  get  any  definite  images, 
because  the  slight  roughnesses  of  the  texture  break 
up  the  wave-fronts  and  scatter  them  in  all  directions. 
That  is  why  a  white  sheet  of  paper  looks  white  from 
whichever  aspect  you  regard  it.  If  the  substance  is  one 
which,  like  silk,  has  a  definite  fibre  or  grain  that  reflects 
a  little  better  in  one  direction  than  in  another,  then  the 
quantity  of  light  reflected  will  depend  partly  upon  the 
direction  in  which  the  grain  catches  the  light,  and  partly 
upon  the  angle  at  which  the  light  is  inclined  to  the 
surface.  This  is  easily  demonstrated  by  examining  the 
appearance  of  a  piece  of  metal  electrotyped  in  exact 
facsimile  of  a  piece  of  silk  fabric.  Here  is  such  a 


I  LIGHTS  AND  SHADOWS  31 

piece.  It  was  deposited1  in  a  gutta-percha  mould  cast 
upon  a  piece  of  figured  silk  brocade  ;  it  reproduces  the 
exact  shimmer  of  silk,  because  it  reproduces  the  grain 
of  the  silk  in  its  operation  of  partial  reflexion.  If  silk 
is  woven  with  warp  of  one  colour  and  weft  of  another, 
the  different  colours  are  better  reflected  at  certain  angles 
—hence  the  effect  produced  by  "shot"  silk. 

To  illustrate  the  property  of  diffuse  reflexion  let  me 
show  you  two  simple  experiments.  Here  is  a  piece  of 
mirror.  Upon  it  I  paint  with  Chinese  white  the  word 
LIGHT.  The  letters  look  white  on  a  dark  background. 
But  if  I  use  it  to  reflect  upon  the  wall  a  patch  of  light 
from  the  electric  lamp  the  letters  come  out  black.  The 
light  that  fell  on  those  parts-was  scattered  in  all  direc- 
tions —  so  those  parts  looked  white  to  you,  but  they 
have  diffused  the  waves  instead  of  directing  them 
straight  to  the  wall  as  the  other  smooth  parts  of  the 
surface  do. 

Let  me  prove  to  you  how  much  light  is  really  reflected 
from  a  piece  of  paper.  I  have  merely  to  shine  my 
lamp  upon  this  piece  of  white  paper,  and  hold  it  near 
the  cheek  of  this  white  marble  bust  for  you  to  see  for 
yourselves  what  an  amount  of  light  it  actually  reflects 
upon  the  object.  Exchanging  the  white  paper  for  a 

1  Made  at  the  Technical  College,  Finsbury,  by  Mr.  E.  Rousseau, 
instructor  in  electro -deposition.  His  process  of  casting,  in  a  molten 
compound  of  gutta-percha,  the  matrices,  which  are  afterwards  metal- 
Ksed  to  receive  the  deposit  in  the  electrotype  bath,  is  distinctly  superior 
to  the  commercial  process  of  taking  moulds  in  a  hydraulic  press. 
On  one  occasion  he  took  for  me  a  mould  of  a  Rowland's  diffraction 
grating,  having  14,400  parallel  lines  to  the  inch.  Like  the  original 
it  showed  most  gorgeous  diffraction  colours. 


LIGHT 


LECT. 


piece  of  red  paper, — that  is  to  say  of  paper  that  reflects 
red  waves  better  than  waves  of  any  other  colour, — and 
you  see  how  the  red  light  is  thrown  back  upon  the 
bust,  and  brings  an  artificial  blush  to  its  cheek. 

If  light  is  reflected  from  one  mirror  to  another  one 
standing  at  an  angle  with  the  first,  two  or  more  images 


may  be  formed,  according  to  the  position  of  the  mirrors. 
Here  (Fig.  18)  are  two  flat  mirrors  hinged  together 
like  the  leaves  of  a  book.  If  I  open  them  out  to  an 
angle  equal  to  one-third  of  a  circle — namely,  120° — and 
then  place  a  candle  between  them,  each  mirror  will  make 
an  image,  so  that,  when  you  peep  in  between  the  mirrors, 
there  will  seem  to  be  three  candles.  If  I  fold  the  mirrors 
a  little  nearer,  so  that  they  enclose  a  quadrant  of  a  circle 


i  LIGHTS  AND  SHADOWS  33 

— or  are  at  right  angles — then  there  will  seem  to  be  four 
candles,  one  real  one  and  three  images.  If  I  shut  the 
angle  up  to  72° — or  one-fifth  of  a  circle — then  there  will 
seem  to  be  five  candles.  Or  to  60° — one-sixth  of  a  circle 
— then  there  appear  six  candles.  This  is  the  principle 
of  the  toy  called  the  Kaleidoscope,  with  which  some  most 
beautiful  and  curious  combinations  of  patterns  and 
colours  can  be  obtained  by  the  multiplication  of  images. 
Even  with  two  such  mirrors  as  these  some  quaint  effects 
are  possible.  When  nearly  shut  up,  a  single  light 
between  them  seems  to  be  drawn  out  into  a  whole 
ring  of  images.  Open  them  out  to  72°  or  to  a  right 
angle,  and  try  the  effect  of  putting  your  two  hands  sud- 
denly between  the  mirrors.  Ten  hands  or  eight  hands 
(according  to  the  angle  chosen)  simultaneously  appear 
as  if  by  magic.  Place  between  the  mirrors  a  wedge  of 
Christmas  cake,  and  shut  up  the  mirrors  till  they  touch 
the  sides  of  the  wedge, — you  will  see  a  whole  cake 
appear. 

It  is  now  time  to  pass  on  to  another  set  of  optical 
effects  which  depend  upon  the  rate  at  which  the  waves 
travel.  I  have  told  you  how  fast  they  travel  in  the  air — 
186,400  miles  a  second,  or  (if  you  will  calculate  it  out 
by  a  reduction  sum)  one  foot  in  about  the  thousand- 
millionth  part  of  one  second.  Well,  but  light  does  not 
go  quite  so  fast  through  water  as  through  air — only 
about  three-fourths  as  fast ;  that  is,  it  goes  in  water  only 
at  the  rate  of  about  138,000  miles  a  second,  or  only 
about  nine  inches  in  the  thousand-millionth  part  of  a 
second.  And  in  common  glass  it  goes  still  slower.  On 
the  average — for  glasses  differ  in  their  composition,  and 

D 


34 


LIGHT 


LECT. 


therefore  in  the  retardation  they  produce  on  light-waves 
—light  only  goes  about  two-thirds  as  fast  as  in  air.  That 
is,  while  light  would  travel  one  foot  through  air,  it  would 
only  travel  about  eight  inches  through  glass. 

Now  as  a  consequence  of  this  difference  in  speed 
it  follows  quite  simply  that  if  the  waves  strike  obliquely 
against  the  surface  of  water  or  of  glass  that  part  of  the 
wave-front  that  enters  first  into  the  denser  medium 
goes  more  slowly,  and  the  other  part  which  is  going  on 
for  a  little  longer  time  though  air  gains  on  the  part  that 
entered  first,  and  so  the  direction  of  the  wave-front  is 
changed,  and  the  line  of  march  is  also  changed.  Let 
us  study  it  a  little  more  precisely.  If  waves  of  light 

proceeding  from  a  point 
P  strike  against  the  top 
surface  of  a  block  of 
glass,  as  in  Fig.  19,  how 
will  the  retardation  that 
they  experience  on  enter- 
ing affect  their  march  ? 
Suppose  that  at  a  certain 
moment  a  ripple  has  got 
as  far  as  FF',  If  it  had 
been  going  on  through 
air  it  would,  after  a  very 
short  interval  of  time,  have  got  as  far  as  GG'.  But 
it  has  struck  against  the  glass,  and  the  part  that  goes 
in  first  instead  of  going  as  far  as  G'  will  only  get 
two-thirds  as  far.  So  once  more  take  your  compasses, 
and  strike  off  a  set  of  arcs  for  the  various  wavelets, 
in  each  case  taking  as  the  arc  two -thirds  of  the  dis- 


FIG.  19. 


I  LIGHTS  AND  SHADOWS  35 

tance  that  the  light  would  have  had  to  go  if  after 
passing  the  surface  it  could  have  gone  on  to  GG'.  The 
overlapping  wavelets  build  up  the  new  wave-front  HG', 
which  you  notice  is  a  flatter  curve,  and  has  its  centre 
somewhere  farther  back  at  Q.  In  fact,  the  effect  of  the 
glass  in  retarding  the  wave  is  to  flatten  its  curvature  and 
alter  its  march,  so  that  in  going  on  through  the  glass  it 
will  progress  as  though  it  had  come  not  from  P,  but 
from  Q,  a  point  i  J  times  as  far  away.  Consider  the 
bit  of  wave-front  that  has  been  marching  down  the  line 
PG'.  When  it  enters  the  glass  its  line  of  march  is 
changed — instead  of  going  on  along  G'A  it  goes  more 
steeply  down  G'B,  as  though  it  had  come  from  Q.  This 
abrupt  change  of  direction  along  a  broken  path,  caused 
by  the  entrance  into  a  denser *  medium,  is  known  by  the . 
term  refraction.  Glass  refracts  more  than  water  does ; 
heavy  crystal  glass  (containing  lead)  refracts  more  than 
the  light  sorts  of  glass  used  for  windowTpanes  and  bottles; 
while  many  other  substances  have  a  still  higher  refrac- 
tivity. 

Now,  we  can  use  this  property  of  the  refracting  sub- 
stances to  produce  convergence  and  divergence  of  light- 
waves, because,  as  you  see,  when  we  want  to  imprint  a 
curvature  on  the  wave-fronts,  we  can  easily  do  this  by 
using  the  retardation  of  water  or  of  glass.  Suppose  we 
wanted  to  alter  a  plane-wave  so  as  to  make  it  converge 
to  a  focus,  what  we  have  got  to  do  is  to  retard  the 
middle  part  of  the  wave-front  a  little,  so  that  the  other 

1  "  Denser,"  in  its  optical  sense,  means  the  same  thing  as  more 
retarding.  Compare  with  what  is  said  on  p.  62  in  the  Appendix  to 
this  Lecture. 


LIGHT 


LECT. 


parts  shall  gain  on  it.  It  will  then  be  concave  in  shape, 
and  therefore  will  march  to  a  focus.  What  sort  of  a 
piece  of  glass  will  do  this  ?  A  mere  window-pane  will 
not.  A  thick  slab  will  not,  seeing  it  is  equally  thick  all 
over.  Clearly  it  must  be  a  piece  of  glass  that  is  thicker 
at  one  part  than  another.  Well,  suppose  we  take  a 
piece  of  glass  that  is  thicker  in  the  middle  than  at  the 
edges,  what  will  it  do  ?  Suppose  that,  as  in  Fig.  20, 
we  have  some  plane-waves  coming  along,  and  that  we 
put  in  their  path  a  piece  of  glass  that  is  flat  on  one  face 


and  bulging  on  the  other  face.  Think  of  the  time  when 
a  wave-front  has  arrived  at  GG.  A  moment  later  where 
will  it  be  ?  The  middle  part  that  strikes  at  M  will  be  going 
through  glass  to  B.  This  distance  MB  we  know  will  be 
only  two-thirds  as  great  as  the  distance  to  which  it  would 
go  in  air.  Had  it  gone  on  in  air  it  would  have  gone  as 
far  as  A,  the  length  MA  being  drawn  ij  times  as  great 
as  MB.  The  edge  parts  of  the  wave-front  go  almost 
wholly  through  air,  and  will  gain  on  the  middle  part. 
So  the  new  wave-front,  instead  of  being  flat  through 
HAH,  will  be  curved  concavely  in  the  shape  HBH ; 


I  LIGHTS  AND  SHADOWS  37 

and  as  a  result  the  wave  will  march  on  converging  to 
meet  at  F  in  a  real  focus.1  It  would  be  the  same  if  the 
piece  of  glass  were  turned  round  the  other  way,  with  its 
bulging  face  toward  the  light ;  it  would  still  imprint  a 
concavity  on  the  advancing  wave  and  make  it  converge 
to  a  focus.  This  is  exactly  how  a  burning-glass  acts. 

With  my  ripple -tank  I  am  able  to  imitate  these 
effects,  but  not  very  accurately,  because  the  only  way  I 
have  of  slowing  the  ripples  is  to  make  the  water  shal- 
lower where  retardation  is  to  be  produced.  This  I  do 
by  inserting  a  piece  of  plate  glass  cut  to  the  proper  shape. 
Where  the  ripples  pass  over  the  edge  of  the  submerged 
piece  of  glass  they  travel  more  slowly.  Where  they  meet 
the  edge  obliquely  the  direction  of  their  march  is  changed 
— they  are  refracted.  Where  they  pass  over  a  lens- 
shaped  piece  they  are  converged  toward  a  focus. 

It  is,  however,  more  convincing  to  show  these  things 
with  light-waves  themselves.  Let  me  first  show  you 
refraction  upon  the  optical  circle  by  the  aid  (Fig.  21)  of 
a  special  apparatus  2  for  directing  the  beam  toward  the 
centre  at  any  desired  angle.  Placing  a  large  optical 
circle  with  its  face  toward  you  and  its  back  to  the  lantern, 
I  can  throw  the  light  obliquely  upon  the  top  surface  of 

1  From  Fig.   20  it  is  easy  to  see  that  the  curvature  of  the  im- 
pressed HAH  is  just  half  (if  MB  =  §  MA)  of  the  curvature  of  the 
glass  surface.      Hence  it  follows  that  the  focal  length  of  the  plano- 
convex lens  (if  of  glass  having  a  refractivity  of  ij)  is  equal  to  twice 
the  radius  of  curvature  of  the  lens-surface.     In  the  case  of  double- 
convex  lenses,  each  face  imprints  a  curvature  upon  the  wave  as  it 
passes  through.     See  Appendix  to  Lecture  I.  p.  65. 

2  This  apparatus,   which  can  be  fitted  to  any  ordinary  lantern, 
consists  of  three  mirrors  at  45°  carried  upon  an  arm  affixed  to  a 


LIGHT 


LECT. 


a  piece  of  glass,  the  under  surface  of  which  has  been 

ground  to  a  semi-cylinder 
(Fig.  22).  The  refracted 
beam  emerges  at  a  differ- 
ent angle,  its  line  of  march 
having  been  made  more 
steeply  oblique  by  the 
retardation  of  the  glass. 
If  you  measure  the  angles 
not  in  degrees  but  by  the 
straight  distances  across 
the  circle,  you  will  find 
that,  for  the  kind  of  glass 

I  am  using,  the  proportion  between  the  length  CD  (the 

sleeve  that  fits  the  condenser-tube,  as  in  Fig.  21.     The  beam  after 
three  reflexions  comes   radially  back  across  the    axis  of  the   con- 


FlG.    21. 


densers  ;  and  by  turning  the  arm  around  in  the  condenser-tube  can 
be  used  at  any  angle. 


I  LIGHTS  AND  SHADOWS  39 

sine  of  refraction)  and  the  length  AB  (the  sine  of  inci- 
dence) is  always  just  the  proportion  of  2  to  3,  whatever 
the  obliquity  of  the  incident  beam.  When  the  incident 
beam  falls  at  grazing  incidence  most  of  it  is  reflected 
and  never  enters  the  glass,  and  the  part  that  does  enter 
is  refracted  down  at  an  angle  known  as  the  critical  or 
limiting  angle. 

With  this  same  optical  circle  I  am  able  to  show  you 
another  phenomenon, 
that  of  total  internal  re- 
flexion. If  I  send  the 
light  upwards  through  the 
glass  hemisphere  (Fig. 
23),  at  an  angle  beyond 
that  of  the  critical  angle, 
none  of  it  will  come  up 
through  the  surface;  all 
will  be  reflected  inter- 
nally at  the  under  side, 

FIG.  23. 

the  top  surface  acting  as 

a  polished  mirror.     You  can  see  the  same  effect  with 

a  tumbler  full  of  water  with  a  spoon  in  it. 

This  same  phenomenon  of  total  reflexion  can  be 
beautifully  illustrated  by  the  luminous  cascade  or  fairy 
fountain.  I  allow  water  to  stream  out  of  a  nozzle,  and 
shine  light  in  behind  through  a  window  into  the  cistern 
from  which  the  water  flows.  It  falls  in  a  parabolic  curve, 
the  light  following  it  internally  down  to  the  place  where 
the  jet  breaks  (Fig.  24)  into  drops. 

Total  reflexion  can  also  be  illustrated  by  shining 
light  into  one  end  of  a  solid  glass  rod,  along  which, 


40  LIGHT  LECT. 

though  it  is  of  a  bent  and  crooked  shape,  the  light 
travels  until  it  comes  to  the  other  end. 

Returning  now  to  the  use  of  lenses  to  cause  the 
waves  to  converge  and  diverge,  we  will  adjust  our  lan- 
tern to  send  out  a  straight  beam,  and  then  interpose  in 


FIG.  24. 

the  path  a  lens  made  of  glass  thicker  in  the  middle 
than  at  the  edges.  At  once  it  is  observed — thanks  to 
the  dust  in  the  air — to  make  these  waves  converge  to  a 
focus  at  F  (Fig.  25).  This  is  again  a  real  focus.  A  lens 
that  is  thus  thicker  in  the  middle  than  at  the  edges  is 
called  a  convex  lens. 

Had  we  taken  a  piece  of  glass  that  is  thinner  in  the 


i  LIGHTS  AND  SHADOWS  41 

middle  than  at  the  edges — a  concave  lens — the  effect 


FIG.  25 


would  be  the  opposite.     Since  the  thin  middle  retards 
the  mid  parts  of  the  wave-front  less  than  the  thick  glass 


FIG.  26. 


edges  retard  the  edge  parts,   the  middle  part  of  the 


42  LIGHT  LECT. 

wave-front  will  gain  on  the  outlying  parts,  and  the  wave 
will  emerge  as  a  bulging  wave,  and  will  therefore  march 
as  if  diverging  from  some  virtual  focus. 

You  will  not  have  failed  to  note  that  this  property  of 
lenses  to  converge  or  diverge  light  depends  on  the  fact 
that  light  travels  slower  in  glass  than  in  air  •  and  you  will 
perhaps  wonder  what  would  be  the  effect  if  there  were 
no  change  in  the  speed  of  travelling.  Well,  that  is  a 
very  simple  matter  to  test.  If  the  action  of  the  lens 
depends  upon  the  difference  of  speed  of  light  in  the 
glass  and  in  the  surrounding  medium,  what  ought  to  be 
the  result  of  surrounding  the  glass  lens  with  some  other 
medium  than  air  ?  Suppose  we  try  water.  The  speed 
of  light  in  water  is  less  than  in  air — it  is  more  nearly 
like  that  in  glass.  And  if  the  action  depends  on  differ- 
ence of  speed,  then  a  glass  lens  immersed  in  water  ought 
to  have  a  less  action  than  the  same  glass  lens  in  air. 
Try  it,  and  you  see  at  once  that  when  immersed  in 
water  a  magnifying  glass  does  not  magnify  as  much  as 
it  does  in  air.  A  burning-glass  does  not  converge  the 
rays  so  much  when  immersed  in  water;  its  focus  is 
farther  away.  Nay,  I  have  here  a  lens  which  you  see 
unquestionably  magnifies.  I  immerse  it  in  this  bath  of 
oil — and  behold  it  acts  as  a  minifying  lens — it  makes 
the  beam  diverge  instead  of  converge !  Carry  the 
argument  on  to  its  logical  conclusion.  If  the  effect  of 
the  medium  is  so  important,  what  would  be  the  effect 
of  taking  a  lens  of  air  (enclosed  between  two  thin  walls 
of  glass)  and  surrounding  it  by  a  bath  of  water  or  oil  ? 
If  the  reasoning  is  right,  a  concave  air  lens  in  oil  ought 
to  act  like  a  convex  glass  lens  in  air,  and  a  convex  air 


LIGHTS  AND  SHADOWS 


43 


FIG.  27. 


lens  in  oil  like  a  concave  glass  lens  in  air.     Let  us  put 

it  to  the  test  of  experiment.     Here  is  a  concave  air  lens. 

In  air  it  neither  converges  nor  diverges  the  light — the 

speed  inside  and  outside  the 

lens  is  the  same — therefore 

there  is  no  action.   But  plunge 

it  in  oil  (Fig.  27)  and,  see,  it 

brings  the  beam  to   a  focus 

(F)  exactly  as  a  convex  glass 

lens  in  air  would  do. 

Let  me  sum  up  this  part 
of  my  subject  by  simply 
saying  that  lenses  and  curved  mirrors  can  change  the 
march  of  light-waves  by  imprinting  new  curvatures  on 
the  wave-fronts.  Indeed,  speaking  strictly,  that  is  all 
that  any  lens  or  mirror,  or  combination  of  lenses  or  of 
mirrors,  can  do. 

Now  the  human  eye,  that  most  wonderful  of  all 
optical  instruments,  is  a  combination  of  lenses  within  a 
cartilaginous  ball,  the  back  of  which  is  covered  on  its 
inner  face  with  an  exquisitely  fine  structure  of  sensitive 
cells,  through  which  are  distributed  ramifications  of  the 
optic  nerve.  All  that  that  nerve  can  do  is  to  feel  the 
impressions  that  fall  upon  it  and  convey  those  impres- 
sions to  the  brain.  All  else'  must  be  done  on  the  one 
hand  by  the  lens-apparatus  that  focuses  the  waves  of  light 
on  the  retina,  or  on  the  other  hand  by  the  brain  that  is 
conscious  of  th e  impressions  conveyed  to  it.  With  neither 
the  nerve-structures  nor  with  the  brain  are  these  lectures 
concerned.  We  have  merely  to  treat  of  the  eye  as  a 
combination  of  lenses  that  focuses  images  on  the  retina. 


44 


LIGHT 


LECT. 


Consider  a  diagram  (Fig.  28)  of  the  structures  of  the 
human  eyeball.  The  greater  part  of  the  refractive 
effect-  is  accomplished  by  a  beautiful  piece  of  trans- 
parent horny  substance  known  as  the  crystalline  lens 
(L2),  which  is  situated  just  behind  the  iris  or  coloured 
diaphragm  of  the  eye.  The  pupil,  or  hole  through  the 
iris,  leads  straight  toward  the  middle  of  this  crystalline 


C  the  cornea. 
R  the  retina. 
N  the  optic  nerve. 
LI  aqueous  humour. 
L2  the  crystalline  lens. 
LS  vitreous  humour. 
i  the  iris  diaphragm. 
b  the  blind  spot. 

y  the  yellow  spot,  or  ma- 
cula lutea. 


FIG.  28. 


lens.  But  it  is  immersed  in  a  medium,  or  rather  between 
two  media,  a  watery  medium  (LJ  in  front  and  a  gelatin- 
ous one  (Lg)  behind;  the  latter  filling  up  the  -rest  of 
the  globe  of  the  eyeball.  The  crystalline  lens  has 
therefore  a  less  magnifying  power  than  it  would  have 
if  it  were  immersed  in  air.  It  acts  very  much  as  a 
lens  in  water.  But  the  watery  liquid  in  front  of  it 
also  acts  as  a  lens,  since  it  occupies  the  space  in  front 
of  the  crystalline  lens  and  between  it  and  the  trans- 


|  t         i    n 

i  LIGHTS  AND  SHADOWS  45 

parent  cornea,  the  bulging  window  of  the  eye.  Taken 
together  these  form  a  lens  -  combination  adapted  to 
form  images  upon  that  back-screen  or  retina,  R,  where 
are  spread  out  the  sensitive  nerve  structures.  All 
that  the  eye  can  do  as  an  optical  instrument  can  be 
imitated  by  optical  combinations  of  lenses.  An  ordinary 
photographic  camera  may  be  regarded  as  a  sort  of 
artificial  eye.  In  front  is  a  combination  of  lenses  the 
function  of  which  is  to  focus  images  upon  a  back  screen, 
or  upon  a  plate  which  is  made  chemically  sensitive. 
To  make  the  analogy  more  complete  one  ought  to 
think  of  the  eye  as  a  kind  of  camera  in  which  the 
hollow  body  is  rilled  up  with  a  thin  transparent  watery 
jelly,  and  in  which  also  the  space  between  the  front 
lens  and  the  one  behind  it  is  full  of  water. 

Apart  from  the  complication  introduced  by  the 
watery  and  gelatinous  media,  it  is  very  easy  to  imitate 
the  optical  arrangements  of  the  eye  by  lenses.  Any 
photographic  camera  will  serve  indeed  for  the  purpose. 
Its  lens  combination  throws  upon  the  screen  at  the 
back  real  images  of  the  objects  placed  in  front. 

As  in  the  camera,  so  in  the  eyeball,  the  images 
thrown  on  the  back  are  inverted  images.  If  you  have 
not  thought  of  this  before  it  seems  hard  to  believe  it : 
nevertheless  it  is  true.  You  have  all  your  lives  had  the 
images  inverted.  Your  brains,  while  you  were  yet 
babies  learned  to  associate  the  impression  received  on 
the  lower  part  of  the  retina  with  objects  high  above 
you.  However  you  may  explain  or  doubt,  the  facts 
are  simply  what  they  are :  the  images  are  upside-down 
at  the  back  of  your  eyeball. 


46 


LIGHT 


I-ECT. 


Beside  the  general  proof  afforded  by  camera-images, 
there  are  two  extremely  simple  proofs  of  this  fact.  The 
first  any  of  you  can  try  at  home ;  all  the  apparatus  it 
needs  being  a  common  pin  and  a  bit  of  card.  It 
depends  upon  the  circumstance  that  if  you  hold  a  small 
object  close  to  a  lens  a  shadow  of  it  may  be  cast  right 
through  the  lens  without  being  turned  upside  down. 
Here  is  -a  lens — it  will  form  inverted  images  of  objects 
if  it  focuses  them  on  a  screen.  But  hold  a  small  object 
close  to  the  lens  (Fig.  29)  and  shine  light  through  it ;  the 
shadows  are  actually  cast  right  side  up  on  the  screen. 
Now  take  a  visiting-card  and  prick 
a  pinhole  through  it  with  a  large- 
sized  pin.  Place  this  hole  about 
an  inch  from  the  eye  and  look 
through  it  at  a  white  cloud  or  a 
white  surface  strongly  illuminated. 
Then  hold  the  pin  upright,  as  in 
Fig.  30,  between  the  eye  and  the 
pinhole.  It  may  require  a  little 
patience  to  see  it,  as  the  pin  must 
be  held  exactly  in  the  right  place. 
You  know  you  are  holding  it  with  the  head  up,  yet  you 
see  it  with  its  head  down,  looking  as  in  Fig.  31.  Now 
if  in  the  case  where  you  know  that  its  shadow  is 
being  thrown  upright  on  the  back  of  your  eye  you 
feel  the  shadow  upside  down,  it  follows  that  when  you 
feel  any  image  right  way  up  it  must  really  be  an  in- 
verted image  that  you  are  feeling. 

The  other  proof  has  the  merit  of  being  direct  and 
objective,  but  does  not  succeed  with  every  eye — some 


FIG.  29. 


I  LIGHTS  AND  SHADOWS  47 

persons  have  the  cartilaginous  walls  of  the  eyeballs  too 


FIG.  30. 

thick.     Stand  in  front   of  a   mirror,  close   one   eye  — 

say  the  right — and  hold  a  candle  in  the  hand  on  the 

same  side.     Hold  the  candle  about 

at  the  level  of  the  closed  eye  so  that 

its  light  just  falls  across  the  bridge 

of    the    nose    into    the    open    eye. 

Then  if  you  look  very  carefully  you 

will  see,  right  in  the  extreme  corner 

of  the  eye,  shining  dimly  through 

the  cartilaginous  white  wall,  a  small 


FIG.  31 


image  of  the  candle  flame — and  it  is  inverted.     If  you 


48  LIGHT  LECT. 

raise  the  candle  higher,  the  image  goes  down;   if  you 
lower  the  candle,  the  image  rises. 

Leaving  lenses  let  me  show  you  a  couple  of 
interesting  experiments  depending  on  the  property  of 
refraction  that  we  have  been  discussing.  In  passing 
through  the  earth's  atmosphere  obliquely,  as  they  do 
when  the  sun  is  low  down  near  the  horizon,  the  sun's 
rays  are  refracted,  and  he  seems  to  be  a  little  higher  up 
in  the  sky  than  he  really  is.  Indeed,  under  certain 
circumstances,  the  sun  can  be  seen  above  the  horizon 
at  a  time  when  it  is  absolutely  certain  that  he  has  really 
set ;  his  rays  in  that  case  come  in  a  curved  path  over 
the  intervening  portion  of  the  globe.  Now  the  circum- 
stances in  which  this  can  occur  are  these — that  the 
successive  strata  of  the  air  must  be  of  different 
densities ;  the  densest  below,  next  the  earth,  and  the 
less  dense  above.  To  demonstrate  this  I  will  take  a 
glass  tank  into  which  there  have  been  carefully  poured 
a  number  of  solutions  of  chloride  of  calcium  in  water  of 

different  densities  —  the 
densest  at  the  bottom. 
You  note  that  the  beam 
of  light  sent  into  the 
trough  takes  a  curved 

path  (Fig.  32).  In  fact,  the  light  turns  round  a  corner. 
The  difference  of  refractivity  that  accompanies 
difference  of  density  is  well  shown  by  a  very  simple 
experiment  upon  heated  air.  You  all  know  that  when 
air  is  heated  it  rises,  becoming  less  dense.  You  all 
know  that,  when  cooled,  air  becomes  more  dense,  and 
tends  to  fall.  But  did  you  ever  see  the  hot  air  rising 


LIGHTS  AND  SHADOWS 


49 


from  your  hand,  or  even  from  a  hot  poker?  Or  did 
you  ever  see  the  cold  air  descending  below  a  lump  of 
ice?  This  is  exceedingly  easy  to  show  you.  All  I 
require  is  a  very  small  luminous  point.  We  will  take 
the  light  of  an  arc-lamp,  shining  through  a  small  hole 
in  a  metal  diaphragm  close  to  it,  and  let  it  shine  on  the 
white  wall.  Now  I  let  this  hot  poker  cast  its  shadow 
on  the  screen,  and  you  see  torrents  of  hot  air,  which 
rising,  cast  their  shadows  also.  Here  is  a  lump  of  ice. 
The  cold  air  streaming  down  from  it  casts  its  shadow. 
Even  from  my  hand  you  see  the  hot  air  rising.  A 
candle  flame  casts  quite  a  dense  shadow,  and  when  I 
open  a  bottle  of  ether  you  see  the  ether  vapour — which 
is  ordinarily  quite  invisible — streaming  out  of  the  neck 
and  falling  down.  Even  a  jet  of  escaping  gas  reveals 
itself  when  examined  by  this  method. 

Another  curious  experiment  consists  in  using  as  a 
lens  a  piece  of  glass  which  has  been  ground  so  as  to  be 
curved  only  one  way — say  right  and  left — but  not 
curved  in  the  other  way.  If  this 
lens  is  thicker  in  the  middle  part 
from  top  to  bottom,  as  in  Fig.  33, 
than  it  is  at  the  two  edges,  it  will 
magnify  things  from  right  to  left, 
but  not  from  top  to  bottom ;  hence 
it  produces  a  distortion.  I  throw 

upon  the  screen  the  portrait  of  a  well-known  old  gentle- 
man. Then  if  I  interpose  in  front  of  him  one  of  these 
"cylindrical  "  lenses,  his  face  will  be  distorted.  And  if  I 
then  turn  the  lens  round  the  distortion  will  alternately 
elongate  his  features  and  broaden  them.  There  are 


50  LIGHT  LECT. 

also  cylindrical  lenses  of  another  kind,  thinner  in  the 
middle  than  at  the  edges.  These  produce  a  distortion 
by  minifying. 

Finally,  I  return  to  the  point  which  I  endeavoured 
to  explain  to  you  a  few  minutes  ago,  that  all  that  any 
lens  or  mirror  can  do  is  to  impress  a  curvature  upon  the 
wave-fronts  of  the  waves. 

The  most  striking  proof  of  this  is  afforded  by  that 
now  rare  curiosity  the  magic  mirror  of  Japan.  In  old 
Japan,  before  it  was  invaded  and  degraded  by  Western 
customs,  many  things  were  different  from  what  they 
now  are.  The  Japs  never  sat  on  chairs — there  were 
none  to  sit  upon.  They  had  no  looking-glasses — their 
mirrors  were  all  of  polished  bronze ;  and,  indeed,  those 
interesting  folk  had  carried  the  art  of  bronze-casting  and 
of  mirror  polishing  to  a  pitch  never  reached  in  any 
other  nation  before  them.  The  young  ladies  in  Japan 
when  they  were  going  to  do  up  their  hair  used  to  squat 
down  on  a  beautiful  mat  before  a  lovely  mirror  standing 
on  an  elegant  lacquered  frame.  Fig.  34  is  photographed 
from  a  fine  Japanese  drawing  in  my  possession.  You 
may  have  seen  pretty  little  Yum-yum  in  the  "  Mikado  " 
squat  down  exactly  so  before  her  toilet-table.  Here  (Fig. 
35)  is  one  of  these  beautiful  Japanese  mirrors,  round, 
heavy,  and  furnished  with  a  metal  handle.  One  face 
has  been  polished  with  care  and  hard  labour ;  the  other 
has  upon  it  in  relief  the  ornament  cast  in  the  mould — in 
this  case  the  crest  of  the  imperial  family,  the  kiri  leaf 
(the  leaf  of  the  Paullonia  imperialis)  with  the  flower-buds 
appearing  over  it.  The  polished  face  is  very  slightly 
convex;  but  on  looking  into  it  none  of  you  young 


FIG.  34  — Japanese  Girls  with  Mirrors. 


UNIVERSITY 


';**/?% 


r  LIGHTS  AND  SHADOWS  51 

ladies  would  see  anything  but  your  own  fair  faces,  or  the 
faces  of  your  friends  around  you,  or  the  things  in  the 
room.  Certainly  you  would  see  nothing  of  the  orna- 
ment on  the  back.  It  is  merely — so  far  as  you  or  the 
former  owner  of  the  mirror  is  concerned — a  mirror. 

But  now  take  this  mirror  and  hold  it  in  the  light  of 
the  sun,  or  in  the  beams  of  an  electric  lamp,  and  let  it 
reflect  a  patch  of  light  upon  the  white  wall,  or  upon  a 
screen.  What  do  you  see  ?  Why,  in  the  patch  of  light 
reflected  from  the  front  of  the  mirror,  you  see  (Fig.  36) 
the  pattern  that  is  on  the  back.  This  is  the  extra- 
ordinary "magic  "  property  that  has  made  these  mirrors 
so  celebrated. 

Another  mirror  has  at  the  back  a  circle  in  high  relief, 
with  a  fiery  dragon  in  low  relief  sprawling  around  it. 
The  face  is  beautifully  polished,  and  shows  no  trace  of 
the  pattern  at  the  back.  But  when  placed  in  the  beams 
of  the  arc -lamp  it  throws  a  patch  of  light  on  the  wall,  in 
which  the  circle  stands  out  as  a  brilliant  line,  whilst  the 
dragon  is  invisible.  It  is  quite  usual  for  the  parts  in 
high  relief  to  produce  this  "magical"  effect,  while  those 
in  low  relief  produce  none. 

For  many  years  it  was  supposed  that  these  mirrors 
were  produced  by  some  trick.  But  the  extraordinary 
fact  was  discovered  by  Professor  Ayrton  in  Japan  that 
the  Japanese  themselves  were  unaware  of  the  magic 
property  of  the  mirrors.  It  results,  in  fact,  from  an 
accident  of  manufacture.  Not  all  Japanese  mirrors 
show  the  property  :  those  that  show  it  best  are  generally 
thin,  and  with  a  slightly  convex  face.  It  was  demon- 
strated by  Professor  Ayrton,  and  I  have  since  accumu- 


52                                            LIGHT  LECT. 

lated    some    other   proofs,1    that  the    effect    is   due    to 

extremely    slight    inequalities    of  curvature    of   surface. 

These   arise    accidentally  in   the  process   of   polishing; 

The  mirrors  are  cast  in  moulds.  To  polish  their  faces 


they  are  laid  down  on  their  backs  by  the  workman,  who 
scrapes  them  violently  with  a  blunt  iron  tool,  using  great 
force.  Fig.  37  is  taken  from  a  Japanese  print  in  the 
British  Museum.  During  this  process  they  become 
slightly  convex.  The  polishing  is  completed  by  scouring 

1  These  differences  of  curvature  of  surface  can  be  proved  (i)  by 
aclual  measurement,  in  some  cases  by  spherometer ,  (2)  by  placing 
a  convex  lens  in  front  to  correct  the  general  convexity  and  then 
observing  directly,  as  in  Foucault's  method  for  testing  figure  of 
mirrors  ;  (3)  by  reflecting  in  the  mirror  the  image  of  a  number  of 
fine  parallel  lines,  whose  distortion  reveals  the  inequalities  of  curva- 
ture ;  (4)  by  taking  a  mould  in  gutta-percha,  and  reproducing  the 
polished  surface  by  electrotype,  which  is  then  silvered.  The  silvered 
type  will  act  as  a  magic  mirror.  In  some  cases  the  "silvering" 
wears  off  the  surface  unequally,  remaining  last  on  the  parts  that  are 
slightly  concave.  The  front  then  shows  faintly  to  the  eye  the 
pattern  on  the  back. 


I  LIGHTS  AND  SHADOWS  53 

with  charcoal  and  scrubbing  with  paper,  after  which 
they  are  "  silvered  "  by  application  of  an  amalgam  of  tin 
and  mercury.  Now  during  the  violent  scraping  with  the 
iron  tool  the  mirror  bends,  but  the  thin  parts  yield  more 
under  the  pressure  than  the  thick  parts  do ;  hence  the 
thick  parts  g:t  worn  away  rather  more  than  the  thin 
parts,  and  remain  relatively  concave,  or  at  least  loss 
convex. 

Amongst  the  proofs  that  these  very  slight  inequalities 
of  curvature  can  thus  reveal  themselves  by  imprinting  a 
convergivity  or  a  divergivity  upon  the  reflected  waves, 
let  me  show  you  this  glass  mirror,  silvered  in  front  and 
quite  flat,  but  having  a  star  engraved  on  its  back.  By 
merely  blowing  air  against  the  back  to  bend  it,  the  star 
becomes  visible  in  the  patch  of  light  reflected  from  the 
face.  Here  the  thin  parts  yield  more  than  the  thick 
ones.  Again,  simply  heating  a  piece  of  looking-glass 
locally,  by  applying  a  heated  iron  stamp  to  the  back  of 
it,  will  cause  the  glass  to  expand  in  the  heated  region, 
and  exhibit  the  pattern  of  the  stamp  in  the  patch  of 
light  reflected  on  the  wall  by  the  mirror. 

Lastly,  I  have  to  exhibit  some  magic  mirrors  made  by 
a  former  pupil  of  mine,  Mr.  Kearton — English  magic 
mirrors — which  have  no  pattern  upon  them,  either  back 
or  front,  but  yet  show  images  in  the  light  they  reflect 
upon  the  wall.  Here  is  one  that  shows  a  serpent ;  here 
another  with  a  spider  in  his  web ;  another  with  a  man 
blowing  a  horn.  These  are  made  by  etching  very  slightly 
upon  the  brass  mirror  with  acid  (an  immersion  of  three 
seconds  only  is  ample),  and  then  polishing  away  the 
etched  pattern.  After  polishing  for  twenty  minutes  the 


54  LIGHT  LECT.  i 

pattern  will  have  disappeared  entirely  from  sight.  But 
you  may  go  on  polishing  for  an  hour  more,  and  still 
the  minute  differences  of  curvature  that  remain  will 
suffice, — though  quite  undiscoverable  otherwise — to 
produce  a  magic  image  in  the  patch  of  reflected  light. 
Though  so  excessively  minute  these  differences  of 
curvature  of  the  mirror  print  their  form  upon  the  wave- 
fronts  of  the  light,  and  alter  the  degree  of  convergency 
or  divergency  of  the  beam. 


APPENDIX    TO    LECTURE    I 


General  Method  of  Geometrical  Optics 

THE  method  of  teaching  Geometrical  Optics  upon  the  lines 
of  the  wave-theory,  which  is  the  key-note  to  this  Lecture, 
has  been  followed  systematically  by  the  author  for  fifteen 
years  in  his  regular  courses  of  instruction  in  Optics  to 
students  attending  his  lectures  in  Physics.  The  treatment 
of  the  subject  before  the  audience  attending  the  Christmas 
course  at  the  Royal  Institution,  many  of  whom  were 
juveniles,  was  necessarily  simplified  and  popularised  ;  but 
the  essential  features  of  the  method  remain. 

The  author  also  published  a  brief  notice  of  this  method 
of  teaching  the  subject  in  1889  in  a  paper  entitled  "Notes 
on  Geometrical  Optics,"  read  before  the  Physical  Society 
of  London,  and  printed  in  the  Philosophical  Magazine 
(October  1889,  p.  232). 

As  the  development  of  the  method  in  the  present 
lecture  is  so  slight,  the  author  deems  it  expedient  to  add 
as  an  Appendix  a  few  further  points  showing  the  application 
to  the  establishment  of  formulae  for  lenses  and  mirrors. 
These  are,  in  fact,  established  much  more  readily  on  this 
basis  than  by  the  cumbrous  methods  that  are  consecrated 
by  their  adoption  in  every  text-book  of  Geometrical  Optics. 


Basis  of  the  Method 

In    treating     optics    from     the     new     standpoint,     we 
have  to  think    about    surfaces  instead   of  thinking  about 


56  LIGHT  LECT.  i. 

mere  lines.  Waves  march  always  at  right  angles  to 
their  surfaces ;  a  change  in  the  form  of  the  surface 
alters  the  direction  of  march.  The  wave -surface  is 
to  be  considered  instead  of  the  "  ray."  The  curvature 
of  the  surface  therefore  becomes  the  all-important  con- 
sideration. All  that  any  lens  or  mirror  or  any  system 
oj  lenses  or  mirrors  can  do  to  a  wave  of  light  is  to  im- 
print a  curvatitre  upon  the  surface  of  the  wave.  If  the 
wave  is  initially  a  plane-wave,  then  the  curvature  imprinted 
upon  it  by  the  lens  or  mirror  will  result  in  making  it  either 
march  toward  a  point  (a  real  focus)  or  march  as  from  a 
point  (a  virtual  focus).  If  the  wave  possesses  an  initial 
curvature,  then  all  that  the  lens  or  mirror  can  do  is  to 
imprint  another  curvature  upon  its  surface,  the  resultant 
curvature  being  simply  the  algebraic  sum  of  the  initial  and 
the  impressed  curvatures.  As  will  be  seen,  in  the  new 
method  the  essential  thing  to  know  about  a  lens  or  mirror 
is  the  curvature  which  it  can  imprint  on  a  plane  wave  : 
this  is,  indeed,  nothing  else  than  what  the  opticians 
call  its  "  power  "  ;  the  focal  power  being  inversely  propor- 
tional to  the  so-called  focal  length.  Another  but  less  vital 
point  in  the  method,  is  the  advantage  of  using  instead  of 
the  so-called  index  of  refraction  a  quantity  reciprocally  re- 
lated to  it,  and  here  denominated  the  velocity-constant. 
The  use  of  the  index  of  refraction  dates  from  a  time 
anterior  to  the  discovery  that  refraction  was  a  mere 
consequence  of  the  difference  of  velocity  of  the 
waves  in  different  media.  The  index  of  refraction 
is  a  mere  ratio  between  the  sines  (or  originally  the 
cosecants)  of  the  observed  angles  of  incidence  and  re- 
fraction. The  uselessness  of  clinging  to  it  as  a  founda- 
tion for  lens  formulae  is  shown  by  the  simple  fact  that,  in 
order  to  accomplish  the  very  first  stage  of  reasoning  in  the 
orthodox  way  of  establishing  the  formulae,  we  abandon  the 
sines  and  write  simply  the  corresponding  angles,  as  Kepler 
did  before  the  law  of  Snell  was  discovered.  The  ele- 
mentary formulae  of  lenses  are,  in  fact,  where  Kepler  left 
them.  It  is  now  common  knowledge  that  the  speed  of 
light,  on  which  refraction  depends,  is  less  in  optically  dense 


APR        METHOD  OF  RECKONING  CURVATURE  57 

media  than  in  air.  The  speed  of  light  in  air  is  not 
materially  different  from  one  thousand  million  feet  per 
second,  or  thirty  thousand  million  centimetres  per  second. 
If  we  take  the  speed  of  light  in  air  as  unity,  then 
the  numeric  expressing  the  speed  in  denser  media,  such  as 
glass  or  water,  will  be  a  quantity  less  than  unity,  and  will 
differ  for  light  of  different  wave-lengths.  It  is  here  pre- 
ferred to  take  the  speed  of  light  in  air,  rather  than  in  vacua, 
as  unity,  because  lenses  and  optical  instruments  in  general 
are  used  in  the  air.  The  numeric  expressing  the  relative 
velocity  in  any  medium  is  called  its  "  velocity-constant "  ; 
it  is  the  reciprocal  of  the  index  of  refraction.  The  velocity- 
constant,  for  mean  (yellow)  light,  for  water  is  about  075  ; 
that  of  crown  glass  0-65  ;  that  of  flint  glass  from  o-6i 
to  0-56,  according  to  its  density. 


Method  of  Reckoning  Curvature 

The  Newtonian  definition  of  curvature  as  the  reciprocal  of 
the  radius  has  a  special  significance  in  the  present  method  of 
treating  optics  :  for  some  of  the  most  important  of  lens  and 
mirror  formulas  consist  simply  of  terms  which  are  reciprocals 
of  lengths,  that  is  to  say  of  terms  which  are  curvatures.  The 
more  modern  definition  of  curvature  as  rate  of  change  of 
angle  per  unit  length  of  the  curve  (Thomson  and  Tait's 
Natural  Philosophy,  vol.  i.  p.  5)  is  equivalent  to  Newton's  ; 
for  if  in  going  along  an  arc  of  length  8s,  the  direction 
changes  by  an  amount  80,  the  curvature  is  86/8s.  But  the 
angle  BO  =  8s /r,  where  r  is  the  radius  of  curvature  ;  hence 
the  curvature  =  BsfrSs  =  I  \r. 

There  is,  however,  another  way  of  measuring  curvature, 
which,  though  correct  only  as  a  first  approximation,  is 
eminently  useful  in  considering  optical  problems.  This 
way  consists  in  measuring  the  bulge  of  the  arc  subtended 
by  a  chord  of  given  length. 

Consider  a  circular  arc  AP,  having  O  as  its  centre. 
Across  this  arc  draw  a  chord  PP'  of  any  desired  length. 
The  diameter  AB  bisects  it  at  right  angles  in  M.  The 


58  LIGHT  LECT.  i 

short  line  MA  measures  the  depth  of  the  curve  from  arc 
to  chord.  If  the  radius  is  taken  as  unity  the  line  MA  is  the 
versed-sine  of  the  angle  subtended  at  B  by  the  whole  chord, 
or  is  the  versed-sine  of  the  semi-angle  subtended  at  the 
centre.  In  Continental  works 
it  is  frequent  to  use  the  name 
sagitta  for  the  length  of  this  line 
MA ;  and  as  this  term  is  pre- 
JB  ferable  to  versed -sine,  and  can 
be  used  generally  irrespective  of 
the  size  of  radius,  it  is  here 
adopted.  The  proposition  is 
that,  for  a  given  chord,  the 
sagitta  is  (to  a  first  degree  of  approximation)  proportional 
to  the  curvature.  For  it  follows  from  the  construction 
that 

MA.  MB  =  (PM)2; 

assuming  PM  as  unity, 


-MB-2r-AM    ' 

But,  for  small  apertures,  AM  is  small  compared  with  2r, 
and  may  be  neglected  in  the  denominator,  whence,  to  a 
first  approximation, 

1  i 

2  *  r " 

Twice1  the  sagitta  represents  numerically  the  curvature. 
The  error  is  less  than  one  per  cent  when  the  semi-angle  sub- 
tended at  the  centre  is  10°  ;  less  than  two  per  cent  when 
if.  is  15°;  less  than  five  per  cent  when  it  is  25°. 

If  the  method  of  reckoning  curvatures  by  means  of  the 
sagitta  required  justification,  that  is  afforded  by  the  fact 
that  the  practical  method  of  measuring  the  curvatures  of 

1  Though  the  sagitta  is  numerically  half  the  curvature,  since  all 
the  formulas  of  first  approximation  are  homogeneous  and  of  the  first 
degree  as  regards  sagittae  and  curvatures,  the  numerical  factor  ^  dis- 
appears in  passing  from  sagittae  to  curvatures,  or  vice  versd. 


APP.   METHOD  OF  RECKONING  CURVATURE     59 

lenses  and  mirrors  by  the  spherometer  consists  essentially  in 
applying  a  micrometer-screw  to  measure  the  sagitta  of  the 
arc  subtended  by  a  fixed  chord,  the  diameter  of  the  contact 
circle  drawn  through  the  three  feet  of  the  instrument.  In 
this  case,  as  indeed  in  all  cases  where  accuracy,  not 
approximation,  is  desired,  the  basis  for  calculation  of  the 
correction  exists  in  the  actual  size  of  the  diameter  of  the 
contact  circle,  which  is  a  fixed  parameter  for  all  measure- 
ments made  with  the  instrument.  The  "lens  measurer" 
used  by  opticians  to  test  the  curvatures  of  spectacle-lenses 
is  a  very  simple  micrometer  which  reads  off  directly  the 
sagitta  of  the  curve  against  which  it  is  pressed,  and  indi- 
cates on  a  dial  the  value  in  terms  of  formula  [10]  on  p.  65. 

The  sign  of  the  curvature  remains  to  be  defined.  In  the 
case  of  actual  waves  of  light,  the  sign  adopted  will  be  + 
for  the  curvature  of  waves  which  are  converging  upon  a 
real  focus  ;  —  for  those  which  are  diverging  either  from  a 
luminous  source  or  from  a  virtual  focus.  This  agrees  with 
the  practice  of  the  ophthalmists  and  of  the  opticians,  who 
always  describe  a  converging  lens  as  positive.  A  positive 
lens  is  one  which  imprints  a  positive  curvature  upon  aplajie 
wave  which  traverses  it. 

The  unit  of  curvature,  whether  of  the  wave-surface 
itself  or  of  the  surface  of  any  mirror  or  lens,  will  be  taken 
so  as  to  accord  with  modern  ophthalmic  and  optical 
practice  as  the  dioptrie ;  that  is  to  say,  the  curvature  of  a 
circle  of  one  metre  radius  will  be  taken  as  unity.  The 
dioptrie,  originally  proposed  by  Monoyer  as  the  unit  of  focal 
power  of  a  lens,  was  formally  adopted  in  1875  by  the 
International  Medical  Congress  at  Brussels,  and  its  great 
convenience  has  led  to  its  universal  adoption  for  the 
enumeration  of  the  focal  powers  of  lenses.  That  lens  which 
has  a  focal  length  of  one  metre  is  said  to  have  a  focal  power 
of  one  dioptrie.  In  other  words,  such  a  lens  prints  a 
curvature  of  one  dioptrie  upon  a  plane  wave  which  is 
incident  upon  it.  For  the  present  proposal  to  extend  the 
use  of  the  term  from  focal  powers  (i.e.  imprinted  wave- 
curvatures)  to  the  curvatures  of  curved  surfaces  in  general, 
the  writer  is  responsible. 


6o 


LIGHT 


LECT.    1 


Notation 

In  adopting  a  notation  which  embodies  the  new  method 
it  is  obviously  advisable  to  choose  one  which  lends  itself 
most  readily  to  the  existing  and  accepted  notations.  In 
the  great  majority  of  books  on  optics,  the  recognised 
symbol  for  focal  length  is/;  that  for  radius  of  curvature  r. 
And  in  the  Cambridge  text-books  for  many  years  the 
distances  from  lens  or  mirror  of  the  point-object  and  the 
point-image  have  respectively  been  designated  by  the 
letters  u  and  v.  Now  it  is  the  reciprocals  of  these  which 
occur  in  the  expressions  for  the  curvatures  of  surfaces  or  of 
waves.  The  symbols  adopted  respectively  for  the  four 
reciprocals  are  accordingly  /%  R,  U,  and  V.  The  accepted 
symbol  for  the  index  of  refraction  is  the  Greek  letter  //, ;  for 
the  velocity-constant,  which  is  its  reciprocal,  we  take  the 
letter  h.  The  following  is  a  tabular  statement  of  the 
symbols  and  their  meanings  : — - 


Symbol. 

Meaning. 

Equivalent  in 
Current 
Notation. 

F 

Focal  curvature,  or  Focal  power  oflens  or 
mirror  (  =  dioptrics,  if  metre  is  taken  as 
unit  of  length)  ..... 

1       . 
J      / 

R 

Curvature  of  Surface       .... 

I 
r 

U 

Curvature  of  Incident  wave  ;  i.e.  curva- 
ture which  ii  has  acquired  by  having 
travelled  from  point  of  origin  ("  incident 
focus  ")  to  incidence  .... 

\      , 
I     " 

V 

Curvature  of  Resultant  wave;  i.e.  curva- 
ture with  which  wave  emerges  from 
the  lens  ...... 

I  ; 

h 

Velocity-constant  of  medium  ;  i.e.  velo- 
city of  light  in  that  medium  compared 
with  velocity  in  air  taken  as  unity 

)    i 
(    ^ 

REFRACTION  FORMULA 


61 


Expansion  of  Curvatures 

If  the  curvature  R  of  a  wave  at  any  point  is  known  it  is 
easy  to  calculate  the  curvature  at  any  other  point  at  distance  d 
farther  from  or  nearer  to  the  centre,  the  formula  for  the  new 
curvature  K  being  as  follows  : —  » 


The  +  sign  must  be  taken  where  the  new  point  is  farther 
from  the  centre  than  the  point  for  which  the  curvature  R 
is  specified  ;  the  —  sign  when  it  is  nearer  the  centre.  This 
proposition  is  of  use  in  dealing  with  thick  lenses,  and  with 
thin  lenses  at  a  given  distance  apart. 


Refraction  Formula; 

As  a  preliminary  to  lens  formulae,  it  is  convenient  to 
consider  certain  cases  of  refraction. 

Consider  a  retarding  medium,  such  as  glass,  bounded 
on  the  left  (Fig.  39)  by  a  plane  surface  SS'.  Let  P  be  a 


FIG.  39. 


source  of  waves  incident  on  the  surface,  PM  being  a  line 
perpendicular  to  SS'.  The  wave-fronts,  at  successive  small 
intervals  of  time,  are  represented  by  arcs  of  circles.  At  a 


62 


LIGHT 


LECT.    I 


certain  moment  the  wave,  had  it  been  going  on  in  air, 
would  have  had  for  its  surface  the  position  SAS' ;  the 
curvature  being  measured  by  the  sagitta  AM.  The 
medium,  however,  retards  the  wave,  and  it  will  only 
have  gone  as  far  as  B  instead  of  penetrating  to  A  ;  B 
being  a  point  such  that  BM  =  ^.AM,  where  h  is  the 
velocity-constant  of  the  medium  into  which  the  wave  enters. 
The  curvature  of  the  wave  is  flattened  as  the  result  of  the 
retardation.  Now  draw  a  circle  through  SBS',  and  find  its 
centre  Q.  To  a  first  degree  of  approximation  the  arc  SBS' 
represents  the  retarded  wave-front,  the  set  of  wave-fronts 


FIG.  40. 


from  B  onwards  being  represented  by  the  series  of  arcs 
drawn  from  Q  as  centre.  An  eye  situated  in  the  medium 
on  the  right  of  SS'  will  perceive  the  waves  as  though 
coming  from  Q,  the  (virtual)  point-image  of  P.  Accurately 
the  wave-fronts  should  be  hyperbolic  arcs,  but  if  SS'  is 
small  relatively  to  PM  the  circular  arcs  are  adequate. 
Now  AM  =  £/,  and  BM  =  V.  Hence  the  action  of  the 
plane  surface  upon  the  curvature  (in  this  case  a  divergivity) 
of  the  incident  wave  is  given  by  the  formula 

V=hU  .  .  [2] 

In  the  above  case,  which  should  be  compared  with  Fig. 
19,    p.   34,   the   wave   had   a   negative   curvature.       If  the 


APR  REFRACTION  FORMULA  63 

entrant  wave  has  a  positive  curvature  or  convergence  such 
as  would  cause  it  to  march  to  a  point  P  to  the  right  in  the 
air,  a  similar  set  of  considerations  will  readily  show  that  if 
on  entering  the  flat  surface  of  a  more  retarding  medium  its 
curvature  is  flattened,  it  will  march  to  a  focus  farther  to  the 
right,  the  ratio  of  the  original  and  the  acquired  curvatures 
being,  as  before,  dependent  simply  on  the  relative  velocities  ; 
and  formula  [2]  above  still  holds  good. 

Consider  next  the  wave  emerging  (Fig.  40)  into  air  from 
a  point  P,  situated  in  the  retarding  medium  whose  velocity- 
constant  is  h.  Had  the  wave  been  going  on  wholly  through 
the  denser  medium,  the  wave-front  would  have  been  at 
SAS' ;  but,  being  accelerated  on  emergence  into  air,  it 
reaches  B  instead  of  A.  The  new  curve  SBS'  has  Q  for  its 
centre  ;  that  is  to  say,  the  wave  emerges  from  Q  as  a  virtual 
focus,  its  curvature  being  augmented.  The  sagitta  BM  is 
greater  than  AM  in  the  ratio  of  I  to  h.  Hence  in  this 
case  the  formula  is 

v=\a  ...        [3] 

The  case  of  an  emergent  wave  of  positive  curvature 
leads  to  the  same  formula.  In  the  case  of  either  positive 
or  negative  initial  curvature,  emergence  from  the  retarding 
medium  through  the  plane  surface  into  air  augments  the 
curvature. 

If  a  plane  wave  travelling  in  air  meets  a  bulging  surface 
of  a  more  retarding  medium  such  as  glass,  the  portion  of 
the  advancing  wave  which  first  meets  the  surface  is  re- 
tarded, so  that  the  wave  front  receives  a  depression,  and 
hence  on  entering  the  second  medium  marches  converging 
toward  a  focus.  The  relation  between  the  impressed  focal 
curvature  and  the  curvature  (R)  of  the  surface  is  given  by 
the  formula 

F=R(i-h]  .  [4] 

It  will  be  noted  that  if  the  curvature  of  the  surface  is 
positive  (i.e.  bulging  toward  the  source  of  light),  the  im- 
pressed focal  curvature  is  also  positive.  The  formula, 
therefore,  is  the  same  for  entrant  plane-waves  whether  the 


64  LIGHT  LECT.  i 

surface  be  convex  or  concave,  the  sign  of  F  following  the 
sign  of  R.  For  the  case  of  any  two  media  having  respec- 
tive velocity-constants  hl  and  //2,  the  formula  becomes 


A  plane  wave  traversing  a  medium  with  velocity  h  and 
emerging  through  a  curved  surface  into  air  has  a  curvature 
imprinted  upon  it  that  is  of  opposite  sign  to  that  of  the 
surface  itself.  If  the  wave  travelling  to  the  right  emerges 
through  a  (hollow)  surface  whose  centre  of  curvature  lies  to 
the  right,  the  acquired  focal  curvature  will  have  its  centre 
to  the  left,,  or  will  be  negative  ;  and  its  relation  to  the 
curvature  (R)  of  the  surface  is  given  by  the  rule 


F=. 


As   before,  for  any  two  media   having  respective  velocity- 
constants  kl  and  hy  the  formula  becomes 


which,   in   the   present    case   where   //1</^9,   will    give  F  ot 
opposite  sign  to  R. 

The  cases  in  which  a  wave  possessing  initial  curvature 
passes  through  a  curved  surface  and  acquires  a  resultant 
curvature  may  be  dealt  with,  apart  from  any  further  geo- 
metrical constructions,  by  applying  the  principle  of  super- 
position of  curvatures.  Thus,  take  the  case  of  a  wave 
possessing  initial  curvature  U  entering  from  air  into  a 
medium  having  velocity-constant  //,  and  so  curved  that  the 
focal  power  of  the  curved  surface  is  F.  Then,  as  the  wave 
enters  the  surface  of  the  medium  two  effects  will  occur  : 
its  initial  curvature  will  be  altered  in  the  ratio  of  the  velo- 
cities, and  there  will  be  superposed  upon  it  the  focal  curva- 
ture of  the  surface  ;  or,  in  symbols, 

[7] 


APP.  LENS  FORMULAE  65 

For  an  emergent  wave,  possessing  initial  curvature  U  in  the 
medium,  the  formula  will  be 

^=I-U+FZ  [8] 

Or,  for  the  case  of  a  wave  passing  from  a  medium  of 
velocity-constant  hl  to  another  of  velocity-constant  h^  the 
formula  will  be 

yJ^U+F  .  .  [9] 

*l 

It  is  easy,  however,  to  prove  any  one  of  the  several  cases 
that  may  arise,  without  in  this  way  relying  upon  the 
principle  of  superposition. 


Lens  Formula 

In  the  case  of  a  lens,  the  curvature  F^  imprinted  on 
a  plane  wave  by  entrance  at  the  first  surface  may  be 
regarded  as  an  initial  curvature  of  the  wave  which 
emerges  through  the  second  surface.  Emergence  into  air 
will,  as  shown  above,  alter  the  curvature  by  augmenting  it 
in  the  ratio  of  i  to  h,  and  superpose  upon  it  the  focal 
curvature  F^  due  to  the  second  surface.  Hence  the  whole 
resultant  curvature  F  imprinted  by  a  thin  lens  on  the  plane 
wave  will  be 

i 

But 
and 


whence 


or 


66  LIGHT  LECT.  i 

This  formula  may  be  compared  with  that  in  the  current 
notation, 


Fig.  20  (p.  36),  gives  an  illustration,  in  which  however 
7?x  is  zero,  as  the  first  face  of  the  lens  is  flat. 

In  the  case  of  a  lens  composed  of  a  medium  7/9,  lying 
between  two  other  media  hl  and  hy  the  formula  becomes 


The  general  formula  [10]  for  the  power  of  any  lens 
consists  of  two  factors — one  depending  solely  on  the  shape 
of  the  lens,  the  other  upon  its  material.  The  latter  factor, 

1—r~)  or  /x  -  i,  is  a  mere  numeric ;  whilst  the  former,  being 

the  difference  of  two  curvatures,  is  itself  a  curvature.  If 
the  curvature  thus  determined  by  "shape  solely  is  expressed 
in  dioptrics,  then,  on  multiplying  by  the  numeric  which 
depends  on  the  nature  of  the  material,  the  resultant  power 
of  the  lens  will  also  be  expressed  directly  in  dioptrics.  In 
the  optician's  "lens-measurer"  (p.  58)  the  dial  readings  are 
already  corrected  by  being  multiplied  by  this  numeric,  thus 
obviating  calculation. 

If  the  lens  has  thickness  d,  the  rule  for  expansion  of 
curvature  at  end  of  §  4  above  at  once  gives 

j-,    ~     i*. L_  rT?i 

F**F*+ZF*t£FJl     ' 

or 


Universal  Formula  for  Lenses 

The  principle  of  superposition  at  once  gives  the  universal 
formula  for  all  lenses  bounded  by  identical  media  on  the 
two  sides  : — 

.  .          [14] 


APP.  REFLEXION  FORMULA  67 

or,  in  words,  the  resultant  curvature  is  the  algebraic  sum  of 
the  initial  curvature  and  the  impressed  curvature.  This 
may  again  be  compared  with  the  formula  in  current 
notation  : 

iii 


The  difference  in  sign  attributed  to  the  term  -  arises  from 
conventions  adopted  in  the  two  systems. 


Formula  for  Two  Thin  Lenses  at  a  Distance  Apart 

The  principle  of  expansion  of  curvature  at  once  gives  us 
as  the  equivalent  focal  power, 


where  /^  and  F^  are  the  focal  powers  of  the  first  and  second 
lenses,  and  d  the  distance  between  them.  F  will  be  in 
dioptrics  if  F^  and  F^  are  in  dioptries  and  d  in  metric  units. 
If  the  two  thin  lenses  are  close  together,  the  resultant 
power  is  simply  the  algebraic  sum  of  the  powers  of  the 
separate  lenses.  One  simply  adds  the  dioptries  of  the 
separate  lenses  to  find  the  resultant  dioptries. 


Reflexion  Formula 

The  plane  mirror  (Fig.  41)  has  surface  SMS'.  The 
incident  wave  would  have  had  front  SAS'  at  a  certain 
instant  had  its  path  lain  wholly  in  air.  The  central 
portion  of  the  wave,  which  would  have  reached  A,  travels 
backwards  to  B,  an  equal  distance,  in  the  same  time. 
The  sagitta  BM  of  the  resultant  curvature  is  equal  to 
and  of  opposite  sign  to  the  sagitta  AM  of  the  initial 
curvature  ;  or 

V=-U   .  .  .          [16] 

There  are  two  cases,  equally  simple,  of  convex  and  con- 


68 


LIGHT 


LECT. 


cave  mirrors.  These  are  separately  shown  in  Figs.  1 3  and  1 4 
(pp.  25,  26),  in  both  of  which  the  incident  waves  are  plane. 
Consider  (Fig".  42)  a  plane  wave  which  at  a  certain  instant 
would  have  arrived  at  SAS'  had  its  path  lain  wholly  in  air. 
The  central  portion  of  the  wave  has,  however,  struck  at  M, 
and  marches  backwards  to  B  in  same  time  as  it  would 
have  taken  to  reach  A.  Hence 


or 


BM=:AM, 

BA  =  2AM. 
But  AM  measures  the  curvature  of  the  mirror,  whilst   BA 


FIG.  41. 


FIG.  42. 


measures    the    curvature    impressed    on    the    plane    wave. 
Hence 

F=2.R    .  .  [17] 

In  Cambridge  notation 

I       2 


or 


It  is  equally  easy  to  establish  the  formula  for  the  action 
of  a  curved  mirror  on  a  curved  wave.     The  principle  of 


APP.  REFLEXION  FORMULA  69 

superposition  at  once  leads  to  a  general  formula,  expressing 
the  sum  of  the  two  actions  of  the  mirror  on  the  wave  ;  it 
reverses  its  initial  curvature,  and  then  imprints  a  focal 
curvature  upon  it.  In  symbols, 

y=_U+F  .  .  [18] 

The  application  of  wave  principles  to  find  the  direction 
of  a  refracted  beam  is  best  handled  by  Ampere's  modifica- 
tion of  Huygens's  construction,  as  in  Fig.  43.  •  In  that 
figure  the  dispersion  produced  by  the  difference  between 
the  velocities  of  light  of  different  colours  is  also  illustrated. 


FIG.  43. 

During  the  time  that  light  (of  any  colour)  would  travel  I 
foot  in  air,  red  light  would  travel  about  7^  inches  in  glass, 
and  violet  light  about  7  inches  in  glass.  The  velocity-con- 
stant for  glass  of  this  kind  is  0-625  for  red  waves,  and 
0-583  for  violet  waves.  Let  EF  be  the  surface  of  glass 
(Fig.  43)  at  which  the  wave  is  entering.  It  marches  in  the 
direction  AO.  Consider  the  portion  of  wave -front  that 
arrives  at  O.  If  it  is  regarded  as  setting  up  a  new  set  of 
waves,  these  will  spread  in  circles  according  to  the  velocity. 
Therefore  around  O  as  centre  draw  a  number  of  circles,  one 
with  unit  radius  to  show  how  the  wave  would  have  spread 
in  a  given  time  had  it  spread  all  round  in  air,  one  (a  semi- 


70  LIGHT  LECT.  i 

circle  only)  with  radius  0-625  to  show  how  far  the  red  wave 
would  have  penetrated  in  glass,  one  (also  a  semicircle)  with 
radius  0-583  to  show  how  far  the  violet  wave  would  have 
travelled  in  glass  in  the  same  time.  Now  produce  OA  to 
B,  and  at  B  draw  a  tangent  meeting  the  surface  of  the  glass 
at  E.  From  E  now  draw  as  many  tangents  as  you  can  to 
the  circles,  to  represent  the  wave-fronts.  EC  will  be  the 
wave-front  of  that  part  of  the  light  which  is  reflected  back 
in  the  direction  OC  ;  ER  will  be  the  wave-front  of  the  red 
light  refracted  down  along  the  direction  OR  ;  and  EV  will 
be  the  wave-front  of  the  violet  light  refracted  down  the 
direction  OV. 


\ 


LECTURE  II 

THE  VISIBLE   SPECTRUM   AND   THE   EYE 

Colour  and  wave-length — Rainbow  tints — The  spectrum  of  visible 
colours — Spectrum  made  by  prism — Spectrum  made  by  grating 
— Composition  of  white  light — Experiments  on  mixing  colours 
— Analysis  of  colours — Blue  and  yellow  mixed  make  white,  not 
green  —  Complementary  tints  —  Contrast  tints  produced  by 
fatigue  of  eye — Other  effects  of  persistence  of  vision — Zoetrope 
— Animatograph 

WAVES  of  light  are  not  all  of  the  same  wave-length.  The 
difference  of  size  makes  itself  known  to  our  eyes  as 
colour.  Just  as  the  sounds  of  different  wave-lengths 
produce  in  our  ears  perceptible  differences  in  pitch,  so 
the  lights  of  different  wave-lengths  produce  in  our  eyes 
different  sensations,  which  we  call  colour.  Any  simple 
kind  of  light — I  am  not  speaking  here  of  mixtures — can 
be  described  in  two  ways;  either  (i)  by  stating  the 
colour-sensation  which  it  produces  on  the  eye,  or  (2) 
more  accurately,  by  stating  what  its  wave-length  or  the 
frequency  of  its  vibrations  is.  To  ascertain  the  wave- 
length of  any  particular  kind  of  simple  light  may  not 
be  a  very  easy  matter,  but  when  once  it  has  been 
measured,  the  statement  of  the  wave-length  is  an 
accurate  description. 


72 


LIGHT 


LECT. 


To  begin  then,  here  is  a  table  in  which  I  have  set 
down,  in  their  order  according  to  wave-length,  biggest 
first,  the  various  kinds  of  simple  light  that  are  visible  to 
the  eye. 


TABLE  I. — COLOURS  OF  THE  SPECTRUM 


NAME  OF  COLOUR. 

Wave-length 
in  millionths  of 

Wave-length 
in  millionths  of  a 

an  inch. 

centimetre. 

Extremest  red 

32  '4 

81-0 

Red     . 

26*0 

65-0 

Orange 

23-3 

58-3 

Yellow 

22'0 

55'i 

Green  . 

20'5 

51-2 

Peacock 

IQ'O 

47*5 

Blue     . 

18-0 

44  '9 

Violet  . 

16-0 

40  'o 

Extreme  violet 

14-4 

36-0 

You  will  note  that  the  red  waves  are  about  twenty-six 
millionths  of  an  inch  long  (i.e.  about  ^9^00  of  an  inch), 
while  the  violet  waves  are  a  little  more  than  half  as  great, 
namely,  sixteen  millionths  of  an  inch  in  wave-length  (i.e. 
about  6-oooQ  of  an  inch).  All  the  other  simple  kinds 
of  light  are  of  intermediate  size.  You  will  note  the  names 
of  the  colours.  In  the  list  you  will  find  neither  white 
nor  black,  for  white  (as  I  shall  presently  show  you)  is  a 
mixture  of  all  these  simple  colours,  and  black  is  simply 
the  absence  of  all  light — a  mere  darkness. 

Now  this  set  of  colours  can  be  produced  naturally 
in  their  proper  order  in  several  different  ways.  The 
simplest  way  is  to  take  some  white  light  which  contains 


II  VISIBLE  SPECTRUM  AND  THE  EYE  73 

all  these  colours  mixed  up  together,  and  sort  them  out. 
But  how  ?  That  is  what  I  want  you  to  understand.  In 
nature  we  find  them  sorted  out  in  the  rainbow,  where 
these  tints  stand  side  by  side.  Can  we  make  an  artificial 
rainbow  ?  How  is  a  rainbow  made  ?  Of  the  smiles  of 
Heaven  commingled  with  the  tears  of  Earth,  if  we 
believe  the  poets.1  Of  sunlight  and  raindrops — (is  it 
not  ?) — which  refract  the  light,  and  in  refracting  it  sort 
out  the  different  kinds  of  light,  and  display  them  in  their 
proper  order.  Perhaps  that  is  a  very  incomplete  de- 
scription of  the  operation  of  building  a  rainbow,  but  it  is 
good  enough  to  give  us  a  hint  towards  experiments. 

Here  is  an  optical  lantern,  with  an  electric  arc-lamp 
inside,  a  sort  of  miniature  sun  to  give  us  white  beams  of 
light.  We  let  the  light  pass  out  in  a  fine  straight  beam, 
and  in  that  beam  we  place — to  serve  as  a  sort  of  mag- 
nified raindrop — this^phere  of  water  contained  in  a  thin 
shell  of  glass.  See  the  bow  which  it  casts  back  upon 
the  whitened  screen.  You  can  recognise  the  usual  tints, 
though  they  are  not  so  brilliant  as  in  the  natural  bow. 

But  having  got  our  clue  to  experiment,  let  us  go  on 
farther.  Try  instead  of  the  bulb  a  three-cornered  bottle 
full  of  water.  We  have  now  no  bow.  The  beam  of  light 
is  abruptly  turned  upward  into  a  new  direction,  and  falls 
upon  the  wall  or  ceiling.  But,  though  we  have  lost  the 
shape  of  the  arch  we  have  gained  in  the  development  of 
the  rainbow  hues.  We  have  now  a  brilliantly  coloured, 
though  rather  nebulous,  colour-patch.  Try  again,  and 

1  "We  are  like  Evening  Rainbows,  that  at  once  shine  and 
Weep— things  made  up  of  reflected  splendor  and  our  own  Tears." — 
.$".  T",  Coleridge, 


74  LIGHT  LECT. 

this  time  try  the  effect  of  varying  the  liquid.  Here  is  a 
three-cornered  bottle  full  of  turpentine.  The  angular 
deviation  of  the  colour-patch  has  become  greater,  but  so 
has  the  breadth  of  our  set  of  tints.  Try  oil  of  cinnamon, 
it  is  still  better.  Try  bisulphide  of  carbon,  still  more 
brilliant  though  still  fuzzy  at  the  edges.  Naturally,  one 
begins  to  think  that  if  a  transparent  three-cornered  bottle 
full  of  liquid  will  thus  display  rainbow  effects,  a  three- 
cornered  piece  of  transparent  glass  ought  to  do  the  same. 
So  it  does :  and  so  we  have  arrived  at  the  use  of  the 
well-known  glass  prism  to  produce  a  spectrum  of  colours. 
The  word  spectrum  means  simply  "  an  appearance  "  :  in 
this  case  an  appearance  of  colours — the  colours  sorted 
out  in  their  order.  To  emphasise  the  fact  that  the 
spectrum  is  in  this  case  produced  by  use  of  a  prism,  it  ?s 
sometimes  called  the  "prismatic  spectrum."  In  all 
cases  you  will  have  noticed  that  the  order  of  the  colours 
is  the  same,  and  that  the  red  light  is  always  refracted 
least,  and  the  violet  light  refracted  most.  If  the  refrac- 
tions of  these  colours  were  equal,  the  prism  would  not 
separate  them.  The  difference  of  the  refractions  between 
the  most-refracted  (violet)  and  least-refracted  (red),  of 
the  visible  kinds  of  light,  is  sometimes  called  "  the  dis- 
persion "  of  the  prism. 

We  have  now  got  to  the  stage  of  Newton's  researches, 
but  there  is  one  further  improvement  to  make,  which 
was  indeed  tried  by  him.  Let  us  try  the  effect  of  alter- 
ing the  arrangement  of  our  beam  of  light.  You  see  we 
have  been  using  a  beam  streaming  out  through  a  round 
hole ;  when  nothing  is  interposed  it  falls  in  a  round  spot 
against  the  wall.  Newton  used  sunlight  streaming 


II  VISIBLE  SPECTRUM  AND  THE  EYE  75 

through  a  hole  in  a  shutter.  Well,  let  us  try  the  effect 
of  using  holes  of  different  lizes  and  shapes.  And,  while 
we  are  about  it,  let  us  try  the  effect  of  focusing  on  the 
wall  the  image  of  the  aperture — by  interposing  a  positive 
lens — so  as  to  work  wit?,  a  well-defined  spot  of  light 
instead  of  a  fuzzy  patch. 

We  begin  by  using  round  holes  of  different  sizes, 
which  we  can  try  one  after  the  other.  Now,  interpose 
the  prism — the  best  one  of  those  yet  tried — in  the  path 
of  the  light.  You  see  that  when  the  aperture  used  is  a 
large  circular  hole,  the  colours  overlap  much  near  the 
middle  and  give  a  mixed  effect.  Whereas  when  we  use 
a  smaller  hole,  though  we  have  less  total  light,  the 
colours  are  more  intense,  simply  because  they  overlap 
less.  Well,  then,  let  us  take  the  hint,  and  substitute  for 
the  small  round  hole  a  narrow  slit.  By  employing  a  slit 
with  movable  jaws  (like  a  parallel  ruler)  we  can  adjust 
it  to  be  as  wide  or  as  narrow  as  we  like.  Again,  we  find 
that  if  the  slit  is  too  wide  the  colours  overlap,  while 
with  a  narrow  slit  the  tints  are  more  intense. 

Our  successive  improvements  have  then  led  us  to 
the  following  combination :  a  slit  to  limit  our  beam,  a 
lens  to  focus  the  image  of  the  slit  as  a  fine  white  line 
on  the  screen,  and  a  prism,  which,  in  refracting  the  light 
of  the  lamp,  also  splits  it  up  (Fig.  44)  into  the  various 
colours  of  which  it  is  compounded. 

Perhaps  you  think  I  am  assuming  things  not  yet 
proved  to  describe  the  action  of  the  prism  as  splitting 
up  the  light  of  the  lamp  into  the  colours  of  which  it  is 
compounded.  Well,  I  admit,  the  phrase  "splitting  it 
up  "  is  not  the  best  that  might  be  selected ;  "  sorting  it 


LIGHT 


LECT. 


out "  would  be  a  better  phrase.  But  each  of  these 
phrases  carries  in  its  use  the  assumption  that  the  white 
light  is  a  mixture  that  can  be  split  up  or  sorted  out  into 
simpler  constituents.  That  is  precisely  Newton's  great 
discovery.  White  light,  supposed  down  to  that  time 
to  be  itself  a  simple  thing,  was  found  and  proved  by 
him  to  be  a  mixture.  The  prism  added  nothing  to  the 
white  light,  it  simply  spread  out  the  constituents  in  their 


FIG.  44 

natural  order.  More  than  a  hundred  years  afterwards  the 
great  poet  and  dramatist  Goethe — "  master  of  those  who 
know  " — fought  against  this  idea,  and  threw  the  whole 
weight  of  his  genius  to  demonstrate,  in  his  Farbcnlehrc, 
the  erroneous  nature  of  Newton's  views.  According  to 
him  the  prism  does  not  merely  spread  out  the  simple 
constituents  of  white  light :  it  takes  simple  white  light 
and  adds  something  to  it  which  gives  it  a  tint  of  one 
sort  or  another.  But  it  was  in  vain.  Beautiful  as  many 
of  Goethe's  experimental  researches  were,  his  theory 


II  VISIBLE  SPECTRUM  AND  THE  EYE  77 

died  of  inanition.  To-day  not  a  single  scientific  man, 
even  in  Germany,  holds  Goethe's  theory  of  optics; 
though  his  fame  as  a  poet  stands  immortal. 

Before  we  follow  the  quest  of  those  other  experi- 
ments by  which  Newton's  theory  of  the  compound 
nature  of  white  light  is  established,  let  me  show  you  a 
second  method  of  spreading  out  white  light  into  a  spec- 
trum of  colours.  In  this  case  I  use  no  prism  ;  and  the 
effect  will  not  be  produced  by  refraction  through  any 
transparent  solid  or  liquid.  Instead,  I  employ  the  little 
instrument  which  I  hold  in  my  hand.  It  is  called  a 
"diffraction  grating."  It  is  simply  a  polished  mirror  of 
hard  bronze,  a  little  more  than  two  inches  wide,  across  the 
surface  of  which  there  have  been  ruled  with  a  diamond 
a  large  number  of  parallel  and  equidistant  scratches. 
You  may  think  it  odd  to  call  a  scratched  mirror  a 
"grating."  But  the  fact  is  that  the  properties  it  pos- 
sesses were  originally  discovered  by  the  use  of  gratings 
made  of  fine  wires.  It  would  be  quite  impossible,  how- 
ever, to  make  a  grating  with  wires  as  fine  as  these 
scratches.  When  you  want  to  produce  a  perfect 
diffraction  grating  there  is  nothing  for  it  but  to  rule 
diamond  scratches ;  and  they  must  be  ruled  by  machin- 
ery of  the  utmost  precision.  Over  the  face  of  this  little 
mirror  there  have  been  ruled  about  30,000  parallel 
lines,  and  not  one  of  them  is  a  millionth  of  an  inch  out 
of  its  proper  place.  It  was  ruled  at  Baltimore  on  Pro- 
fessor Rowland's  machine.  The  exact  number  of  lines 
is  14,400  side  by  side  to  the  inch. 

I  set  up  the  grating  so  that  the  light  of  my  lantern, 
issuing  through  the  slit,  falls  upon  it,  and  you  see  the 


78  LIGHT  LECT. 

spectrum  that  it  casts  upon  the  wall.  This  is  not  a 
prismatic  spectrum^  for  there  is  no  prism.  It  is  a  diffrac- 
tion spectrum ;  and  that  is  not  quite  the  same  thing. 
As  a  matter  of  fact  the  grating,  as  you  see,  .casts  on  the 
wall  a  whole  series  of  spectra.  It  reflects  back  centrally 
a  white  image  of  the  slit.  Right  and  left  we  have  on 
each  side  a  bright  spectrum  with  all  the  colours.  Then, 
still  farther  away  on  each  side,  a  rather  longer  and 
nearly  equally  brilliant  spectrum  of  the  second  order ; 
while,  more  dimly,  and  slightly  overlapping  one  another, 
we  have  spectra  of  the  third  and  fourth  orders.  We 
will  deal  only,  however,  with  the  first  bright  spectrum. 
There  are  our  rainbow  tints  in  their  order  as  before. 
But  note  that  now  it  is  the  red  light  that  seems  to  have 
been  turned  most  aside,  and  the  violet  light  which  is 
least.  Note,  further,  that  while  the  order  of  the  colours 
between  red  and  violet  is  the  same  as  in  the  prismatic 
spectrum,  the  spacing  of  them  is  not  the  same.  In  the 
prismatic  spectrum  the  orange  is  huddled  up  toward 
the  red,  and  the  yellow  toward  the  orange;  while  the 
violet  and  blue  are  highly  elongated.  In  the  diffrac- 
tion spectrum  the  red  end  is  not  squeezed  together  un- 
duly, nor  the  violet  end  unduly  drawn  out. 

Time  will  not  allow  me l  to  dwell  on  the  reasons  for 
these  differences.  Suffice  it  to  say  that  they  depend 
upon  the  wave-lengths  of  the  different  kinds  of  light, 
and  their  relations  on  the  one  hand  to  the  size  of  the 
molecules  of  the  refracting  prism,  and  on  the  other  hand 
to  the  width  of  the  bars  of  the  grating. 

Incidentally  you  may  be  interested  in  knowing  that 
1  See  Appendix,  p.  100. 


II  VISIBLE  SPECTRUM  AND  THE  EYE  79 

this  property  of  diffraction,  which  belongs  to  the  surface 
that  has  thus  been  covered  with  parallel  scratches,  can 
be  transferred  from  the  grating  to  another  surface  by 
merely  taking  a  cast.  Here  is  a  cast  made  in  gutta- 
percha  1  from  the  grating ;  it  is  itself  a  grating.  Like  the 
bronze  original  it  glitters  with  rainbow  tints,  and  will 
throw  a  set  of  spectra  on  the  wall.  Mother-of-pearl 
glitters  with  rainbow-tints  for  precisely  the  same  general 
reason,  it  possesses  naturally  a  structure  of  fine  striations 
or  ridges  which  produce  (rather  irregularly)  diffraction. 
But  as  the  ridges  are  not  quite  equidistant,  the  tints  are 
never  pure.  But,  do  you  know,  if  you  will  take  with 
sealing  wax — black  wax  is  best — an  impression  from  a 
piece  of  mother-of-pearl,  you  will  find  it  glitter  just  as 
the  mother-of-pearl  does. 

Whichever  of  these  two  means  we  use — prism  or 
grating — of  producing  a  spectrum,  you  will  note  that 
what  we  do  is  to  sort  out  the  mixture  into  its  con- 
stituents;  we  analyse  the  light,  presently  we  shall  be 
able  to  make  use  of  this  sorting  process  to  discover  what 
some  of  the  compound  colours  are  made  up  of.  But  in 
the  meantime  we  will  return  to  the  prismatic  method  to 
show  some  further  experiments. 

To  produce  a  good  arched  rainbow  artificially,  but  in 
all  the  splendour  of  the  natural  colours,  I  have  recourse 
to  a  specially  constructed  compound  conical  prism.  A 
glass  cone  of  light  crown  glass  is  mounted  with  its  point 
turned  inward  (Fig.  45),  within  a  hollow  truncated  cone 
of  glass,  the  face  of  which  is  closed  with  a  glass  plate. 
The  annular  space  is  filled  with  a  highly  refracting  liquid, 
1  Made  by  Mr.  E.  Rousseau  ;  see  footnote,  p.  31  ante. 


8o 


LIGHT 


LECT. 


cinnamic  ether.1     An  annular  slit  in  a  plate  of  tin-foil  is 
fixed  against  the  end  of  the  cone  ;  and  the  whole  prism 


FIG. 


is  placed  in  a  nearly  parallel  beam  of  light  issuing  from 
the  lantern.     Beyond  it   is   a  lens  to  focus   the   light. 

1  This  liquid  is  excellent  for  direct-vision  prisms,  for  it  has 
exactly  the  same  mean  refractive  index  as  one  kind  of  the  light 
crown  glasses  made  at  Jena.  Figs.  46  and  47  depict  the  direct 
vision  prism  with  parallel  end-faces  designed  by  the  author  in  1889, 
and  constructed  by  Messrs.  R.  and  J.  Beck.  A  prism  A  of  this  glass 


FIG.  46. 


FIG.  47. 

with  a  refracting  angle  of  135°  is  immersed  in  a  glass  cell  filled  with 
cinnamic  ether.  Yellow  light,  as  shown  in  Fig.  47,  goes  straight 
through,  while  red  light  is  thrown  to  one  side  and  violet  to  the  other. 
This  prism  is  very  suitable  for  projecting  the  spectrum  on  the  screen. 


VISIBLE  SPECTRUM  AlS^SEEsBYE  81 


Thus  we  project  upon  the  white  screen  in  almost  exactly 
true  proportions  a  rainbow.  Note  the  order  of  the 
colours,  red  along  the  outer  edge,  then  orange,  a  trace 
of  yellow,  green,  peacock,  blue,  and  lastly  violet  along 
the  inner  edge.  This  is  the  cprrect  order  as  in  the 
natural  rainbow.  But  you  often  see  it  incorrectly  de- 
picted by  artists — they  put  the  colours  in  the  wrong 
order,  or  with  the  red  along  the  inner  edge  and  the 
violet  along  the  outer. 

My  assistant  will  now  give  us  upon  the  screen  the 
bright  spectrum  which  we  saw  before,  in  order  that  we 
may  study  the  effects  of  the  different  kinds  of  light  on 
coloured  stuffs.  Here  is  a  piece  of  blue  drapery,  and 
here  a  piece  of  scarlet.  What  is  the  effect  of  putting 
these  into  the  spectrum,  first  into  one  kind  of  light,  and 
then  into  another?  If  we  put  the  blue  stuff  into  the 
red  or  orange  or  yellow  light  it  looks  simply  black.  But 
in  the  blue  part  of  the  spectrum  it  looks  blue,  in  the 
violet  part  it  looks  violet,  in  the  green  part  it  looks 
green.  Clearly  the  surface  of  it  is  incapable  of  reflect- 
ing back  either  red,  orange,  or  yellow,  while  it  is  cap- 
able of  reflecting  back  green,  blue,  and  violet.  In  fact, 
when  ordinary  daylight  falls  on  it,  it  absorbs  some  of 
the  waves  and  destroys  them,  while  it  reflects  back  to 
our  eyes  some  others  of  the  waves,  and  give's  us  on  the 
whole  a  blue  effect  from  the  green,  blue  and  violet 
waves  mixed  together  and  reflected  back.  The  red 
stuff,  when  placed  in  the  red  part  of  the  spectrum,  looks 
red,  and  in  the  orange  and  yellow  parts  it  looks  dull 
orange  and  dull  yellow ;  while  in  all  the  other  parts  of 
the  spectrum  it  looks  black.  This  red  stuff  then  absorbs 

G 


82 


LIGHT 


LECT. 


and  suppresses  all  the  violet,  blue,  peacock,  and  green 
rays,  and  reflects  back  only  those  at  the  red  end  of  the 
spectrum.  But,  of  course,  it  would  only  look  red  if 
there  was  some  red  light  present.  And  the  blue  stuff 
would  only  look  blue  if  there  was  some  blue  light  pre- 
sent. The  colour  is  really  not  in  the  stuff,  it  is  in  the 
light  that  the  stuff  reflects. 

To  prove  this  let  us  see  how  these  red  and  blue  stuffs 
appear  when  we  shine  upon  them 
some  light  that  has  neither  red,  nor 
blue,  nor  green,  nor  violet  in  it,  but 
has  yellow  only.  The  monochro- 
matic lamp  which  Professor  Tyndall 
used  to  employ  here  (Fig.  48)  has 
been  lit.  It  consists  of  an  atmo- 
spheric gas-burner,  into  the  dim 
flame  of  which  salt  is  projected,1 
making  a  splendid  yellow  flame 
devoid  of  every  other  kind  of  light. 
I  hold  these  blue  and  red  stuffs  in 
the  light  of  the  yellow  flame.  The 
one  appears  simply  black,  the  other 
a  dull  gray.  A  set  of  stripes  of 
gay  colours  painted  upon  a  board 
appear  simply  dull  grays  and  blacks,  except  the  yellow 
stripe,  which  seems  brighter  than  all  the  others.  Even 
gaily-coloured  flowers  seem  merely  black  or  gray  ;  while 

1  The  salt  is  contained  in  an  annular  pan  at  the  top  of  an  external 
tapering  chimney  of  sheet  iron.  This  annular  pan  has  a  gauze 
bottom,  through  which  on  tapping  the  chimney  the  salt  falls  in  fine 
powder  into  the  flame. 


FIG.  48. 


ii  VISIBLE  SPECTRUM  AND  THE  EYE  83 

the  complexion  of  the  human  countenance  appears 
simply  ghastly. 

Now  if  Newton's  view  is  correct  that  white  light 
consists  of  the  lights  of  various  colours  mixed  up 
together,  it  ought  to  be  possible  to  make  white  light 
by  taking  lights  of  all  the  various  colours  and  mixing 
them  together.  Do  not  try  to  mix  together  pigments 
out  of  your  paint  box — they  won't  make  white  paint 
when*  mixed.  That  is  because  pigments  are  not  lights 
— they  are  darknesses  rather  than  lights.  Think  for  an 
instant  what  you  do  when  you  want  to  paint  a  card 
crimson.  You  take  a  piece  of  white  card,  and  paint  over 
it  a  pigment  which  darkens  it,  so  that  it  sends  back  to 
your  eyes  crimson  only,  and  absorbs  the  other  parts  of 
the  white  light.  No,  you  must  not  mix  pigments — you 
must  mix  lights,  and  mix  them  in  the  correct  proportions. 

Now  there  are  several  ways  of  doing  this ;  and  first 
of  them  we  will  take  the  spectrum  colours  'and  recom- 
bine  them  to  produce  white  light.  We  take  the  spec- 
trum light  as  it  issues  obliquely  from  the  prism,  and 
reflect  it  upon  the  screen  with  a  piece  of  silvered  mirror- 
glass.  By  simply  waggling  the  mirror -glass  upon  its 
stand,  we  cause  the  spectrum  to  oscillate  rapidly  across 
the  screen.  The  colours  then  all  blend  by  rapid  super- 
position, and  we  obtain  a  white  band  bordered  by  colour 
only  at  the  ends. 

Another  way  to  recombine  the  spectrum  is  to  employ 
a  cylindrical  lens  (Fig.  33,  p.  49),  so  placed  in  the  path  of 
the  diverging  coloured  rays  that  it  collects  them  back 
to  a  focus  on  the  screen,  and  gives  us  back  the  image  of 
our  slit  as  a  white  streak. 


84 


LIGHT 


LECT. 


An  independent  plan,  suggested  by  Newton,   is  to 

paint  upon  a  circular  card1 
(Fig.  49),  in  narrow  sec- 
tors, the  various  tints  in 
proportions  ascertained  by 
experiment  to  give  the  best 
result;  and  then,  putting 
this  upon  a  small  whirling- 
table,  spin  it  round  so  fast 
that  the  colours  all  blend 
in  the  eye,  giving,  when 
well  illuminated  against  a 
black  background,  the 
effect  of  white.  A  similar 
arrangement  can  be  made 
for  use  in  the  lantern,  the 
sector-disk  being  painted 
in  transparent  tints,  or 

coloured  by  affixing  narrow  wedges 

of  coloured  transparent  gelatine. 
This   method    of   colour  -  mixing 

by  whirling   round  before   the    eye 

surfaces    tinted    with     the     colours 

desired  to  be  mixed,  is  capable   of 

extension  to  other  cases.     Suppose 

we  wish,  for  example,  to  mix  red  and 

green,  or  blue  and  orange  together,  we  have  only  to  paint 

1  The  Colour-whirler  actually  shown  was  lent  by  Messrs.  Harvey 
and  Peak,  who  use  strips  of  brilliantly  tinted  paper  pasted  upon  a 
card  in  such  a  way  as  to  repeat  the  gamut  of  colours  from  red  to 
violet  five  times  around  the  circle.  If  the  colours  are  thus  repeated 
the  card  does  not  require  to  be  whirled  very  fast  to  produce  white. 


FIG.  50 


VISIBLE  SPECTRUM  AND  THE  EYE 


a  round  disk  (Fig.  50)  with  the  colours  desired  to  be 
mixed,  either  in  semicircles,  quadrants,  or  in  any  other 
desired  proportion,  and  place  them  upon  a  whirling 
machine  to  see  the  effect. 

The  arrangement  which  I  now  show  offers  an  im- 
provement in  several  respects.  Upon  the  whirling- 
table  is  fixed  a  light 
cylinder  of  wood, 
which  is  slightly 
tapered  toward  the 
top,  so  that  over  it 
may  easily  be  slipped 
a  paper  sleeve  or 
tube  upon  which  the 
colours  are  painted. 
I  have  here  several 
of  these  paper  tubes. 
The  colours  (in  most 
cases  coloured  paper 
being  cut  to  shape 
and  pasted  on)  to  be 
mixed  are  arranged 
in  two  sets  of  narrow 
triangles,  as  shown  in  the  figure.  When  these  are 
whirled,  one  gets  combinations  in  all  proportions. 
For  instance,  if  red  and  green  are  the  two  colours 
chosen,  one  end  of  the  revolving  surface  is  full  red, 
the  other  full  green,  and  the  colours  gradually  fade 
one  into  the  other.  About  the  middle,  one  obtains 
a  curious  gray,  which  if  seen  by  daylight  looks  rather 
greenish  ;  but  by  gas-light  or  lamp-light  looks  rather 


86  LIGHT  LECT. 

reddish  owing  to  the  greater  relative  prevalence  of  red 
waves  in  artificial  light.  This  change  of  apparent  tint 
is  similar  to  that  observed  in  the  rare  gem  called  the 
alexandrite,1  which  is  green  by  day  and  deep  red  by  night. 
Returning  from  the  operations  of  colour-mixing  by 
rotation,  I  return  to  the  property  of  the  prism  to  analyse 
mixed  lights  by  spreading  out  the  constituent  colours  as 
a  spectrum.  Newton  tried  an  experiment  to  see  whether 
if  you  took  light  of  one  tint  alone  you  could  split  it  up 
still  further  by  passing  it  through  a  second  prism.  I 
introduce  across  the  path  of  the  spectrum  on  its  way  to 
the  screen  a  diaphragm  of  cardboard,  having  a  narrow 
slit  in  it.  I  push  it  along  so  that  the  slit  allows  waves  of 
but  one  particular  colour — say  green — to  pass.  Now,  if 
I  interpose  beyond  this  slit  a  second  prism,  I  find  that 
it  turns  the  beam  of  green  light  round  at  an  angle,  and 
widens  it  out  a  little  more,  but  it  does  not  split  it  up  into 
any  other  colours :  it  is  still  a  green  beam.  So  it  would 
be  with  any  other.  When  once  you  have  procured  a 
simple  tint  by  dispersing  away  the  other  colours  to  right 
and  left,  the  prism  effects  no  further  analysis  2  of  colour. 

1  A  gem  of  the  emerald  species  found  in  a  mine  in  Siberia  be- 
longing to  the  Imperial  Russian  family. 

2  In  this  sense  every  pure  spectrum  tint  is  a  primary  tint,  and  the 
number  of  such  tints  incapable  of  further  analysis  is  infinite.     Each 
kind  of  light  of  a  given  wave-length  is  thus  a  simple  tint.     But  the 
eye  possesses  three  different  sensations  of  colour,  each  of  which  is 
physiologically  a  primary  sensation.     These  three  primaries  are,  a 
red,  a  rather  yellowish  gi'een,  and  a  blue-violet.     Any  other  tint  than 
these  excites  more  than  one  sensation.     For  instance,  a  pure  spec- 
trum yellow  excites  both  the  red  and  the  green  sensations ;  there- 
fore yellow  cannot  be  called  truly  a  primary.     In  the  same  way 
peacock  tint  excites  the  green  and  the  blue-violet  sensations. 


n  VISIBLE  SPECTRUM  AND  THE  EYE  87 

Now  let  us  try  a  few  experiments  in  the  analysis  of 
colours  by  the  prism.  There  are  many  well-recognised 
tints,  known  by  familiar  names,  which  are  not  to  be  seen 
in  the  simple  colours  of  the  spectrum,  for  the  simple 
reason  that  they  are  compound  colours.  In  the  spec- 
trum there  is  no  purple ;  for  purple  is  a  mixture  of  red 
from  one  end  of  the  spectrum  with  violet  or  blue  from 
the  other  end.  Pink  does  not  exist  in  the  spectrum — 
for  pink  is  red,  diluted  by  admixture  with  white,  that  is 
to  say,  with  a  little  of  every  other  colour.  Neither  is 
there  any  chocolate  colour,  which  is  red  or  orange 
diluted  with  black,  that  is  to  say,  a  little  red  or  orange 
spread  where  there  is  no  light  of  any  other  colour. 
Buff,  olive,  russet,  bistre,  slate,  and  many  other  colours 
are  also  compounds.  Well,  whatever  they  are,  the  pnsm 
can  analyse  them.  Here  is  a  piece  of  gelatine,  such  as 
you  may  get  off  a  Christmas  cracker,  stained  a  beautiful 
purple.  Why  does  it  look  purple  ?  What  kinds  of 
light  does  it  actually  allow  to  pass  through  it  that  it 
should  look  purple  ?  I  have  merely  to  interpose  it  in 
the  path  of  the  white  light  for  you  to  see  the  beautiful 
purple  colour  on  the  screen.  Now  placing  the  prism 
in  front  of  it  you  see  the  purple  spread  out  into  its. 
constitutents.  There  is  red  at  one  end ;  there  are 
violet  and  blue  at  the  other.  But  in  between,  where 
orange,  yellow,  green,  and  peacock  colours  should  come, 
there  is  darkness.  The  purple  stain  in  the  gelatine  cuts 
off  all  these  and  lets  the  others  go  by.  Here  is  another 
piece  of  gelatine  stained  with  magenta — you  see  it  lets 
more  red  and  a  little  violet  and  blue  go  through.  Here 
is  a  small  glass  tank  containing  the  pale  purple  liquid 


88 


LIGHT 


known  as  Condy's  fluid  (permanganate  of  potash) ;  on 
interposing  it  across  the  beam  of  light  through  the  prism 
you  see  (Fig.  52)  that  it  cuts  off  the  yellow  and  greenish 
yellow,  but  transmits  red  and  orange  at  one  end  of  the 
spectrum,  and  at  the  other  violet,  blue,  peacock,  and 
some  green.  Here  are  some  coloured  liquids  in  bottles 
(Fig.  53) ;  red  liquid  (amyl  alcohol  dyed  with  aniline- 
red)  floating  on  the  top  of  a  green  liquid  (cupric 


FIG.  52. 


FIG.  53. 


chloride  dissolved  in  dilute  hydrochloric  acid)  without 
mixing.  The  red — as  you  see  when  I  expose  it  to 
analysis  in  the  spectrum — is  a  good  red — it  cuts  off  every 
tint  except  red.  The  green  is  also  a  fairly  good  green — 
it  cuts  off  everything  except  green,  peacock,  and  a  trace 
of  blue.  What  will  happen  if  I  now  shake  up  the  two 
solutions  and  mix  them  ?  I  obtain  a  mixed  liquid l  that 
cuts  off  everything,  and  is  simply  black.  Many  other 
experiments  of  an  instructive  kind  may  be  tried  with 

1  The  liquids  named  possess  the  very  convenient  property  of 
separating  from  one  another  in  a  very  few  minutes.  In  preparing 
the  experiment  a  little  trouble  and  care  is  required  to  get  the  solu- 
tions to  balance.  By  adding  first  a  little  of  the  red  liquid  and  then 
a  little  green  as  may  be  required,  and  trying  the  effect  of  shaking 
up,  the  liquids  may  be  adjusted.  Various  other  colour-combinations 
are  possible  in  this  v/ay. 


II  VISIBLE  SPECTRUM  AND  THE  EYE  89 

coloured  liquids,  their  apparent  tints  depending  on  the 
kinds  of  light  they  absorb  and  transmit  respectively. 
Any  liquid  which  merely  absorbs  green  will  look  red- 
dish, since  in  the  balance  of  colours  it  transmits,  the 
complementary  red  will  preponderate.  Similarly  any 
liquid  (or  glass)  which  merely  absorbs  the  blue  part  of 
the  spectrum  will  look  yellow.  It  is  even  possible  to 
find  a  liquid,1  which  though  it  looks  yellow  to  the  eye 
really  transmits  nothing  but  green  and  orange,  which 
when  mixed  have  the  same  effect  on  the  eye  as  yellow. 
This  proves  that  the  sensation  of  yellow,  though  it  may 
be  excited  by  a  simple  spectrum  tint  of  a  particular 
wave-length,  can  also  be  excited  by  a  mixture  of  other 
tints,  and  is  therefore  not  a  primary  colour-sensation  as 
red,  green,  and  blue-violet  are. 

And  this  brings  me  to  another  point,  viz.  that  while 
yellow  light  can  be  thus  made  by  mixing  together 
orange  and  green  lights,  it  is  found  to  be  absolutely 
impossible  to  produce  green 2  by  mixing  together  any 
two  other  pure  lights.  Blue  light  and  yellow  light,  as 
remarked  above,  do  not  when  mixed  produce  green, 
but  white.  This  is  so  fundamental  a  matter  that  it  is 
worth  while  to  illustrate  it  by  further  experiment. 

My  assistants  have  two  lanterns.  From  each  of 
them  there  is  now  thrown  upon  the  screen  a  round 
white  disk  of  light.  In  front  of  one  lantern  is  interposed 
a  film  of  blue  gelatine — and  that  disk  turns  unmistak- 

1  Mixed  solutions  of  chromic  chloride  and  potassium  bichromate. 

2  Just  as  also  it  is  impossible  to  produce  red  light  by  mixture  of 
any  other  two  simple  lights,  or  to  produce  blue-violet  by  admixture 
of  any  other  two  simple  lights.     These  three — red,  green,  and  blue- 
violet  being  the  three  primary  colour  sensations. 


90  LIGHT  LECT. 

ably  blue.  In  front  of  the  other  lantern  is  interposed 
a  film  of  yellow  gelatine,  and  the  second  disk  of  light 
on  the  screen  becomes  bright  yellow.  Now  one  of  the 
lanterns  is  turned  a  little  aslant  so  as  to  make  one  of 
the  disks  overlap  the  other  (Fig.  54).  Where  they  over- 


FIG.  54 

lap  and  the  lights  mix  we  have — not  green — but  white  ! 
I  put  in  the  lantern  a  colour-whirler,  having  a  disk 
covered  over  half  with  blue  and  half  with  yellow 
gelatine,  and  on  whirling  it  round  the  blue  and  yellow 
mix,  and  make  white. 

Here  is  an  experiment  that  any  boy  might  make  at 
home.  A  cardboard  disk  is  divided 
into  twelve  sectors,  six  of  which  are 
covered  with  blue  paper,  and  the 
alternate  six  with  yellow  (Fig.  55). 
I  put  a  pin  through  the  centre,  and 
spin  it  round  by  hand — and  behold 
blue  and  yellow  are  mixed,  and 
FlG' 55'  make  white. 

We  give  the  name  of  "  complementary  "  tints  to  any 


II  VISIBLE  SPECTRUM  AND  THE  EYE  91 

pair  of  tints  which  thus  mixed  together  make  white. 
That  is  to  say,  if  any  two  tints  are  so  related  that  each 
contains  the  constituents  that  are  wanting  in  the  other, 
then  we  describe  them  as  the  complement  one  of  the 
other.  Here  is  a  table  of  some  tints  which  experience 
shows  to  be  complementary  one  to  the  other : — 

TABLE  II. — COMPLEMENTARY  TINTS 


Crimson  is  complementary  to 
Scarlet                   „ 

Moss  green 
Peacock 

Orange                  ,, 
Yellow 

Turquoise 
Blue 

Primrose               „ 

Violet 

Green-yellow        ,, 

Purple 

There  are  other  cases  also  not  set  down  on  the  list. 
Now  seeing  that  the  sensation  of  white  is  not  excited 
unless  all  three  primary  sensations l  (red,  green,  violet), 
are  stimulated  at  the  same  time,  it  is  clear  that  when 
two  colours  are  found  that  are  complementary  to  one 
another,  by  no  possibility  can  both  be  primary  colours. 
One  may  be,  but  in  that  case  the  other  will  be  a 
mixture  of  the  two  other  primaries.  If  primary  r^d  is 
one  of  the  two  complementary  tints  the  other  will  be  a 
bluish -green  or  peacock  colour  made  up  of  primary 
green  and  primary  blue-violet  mixed. 

Probably  most  of  you  are  aware  of  the  subjective 
colours  that  are  seen  on  closing  the  eyes  after  looking 

1  The   red   that   is   primary  is  a  full   red.     The   green   that   is 
primary  a  rather  yellowish-green,  the  violet  a  rather  bluish-violet. 


LIGHT 


LECT. 


at  a  bright  light.  These  are  connected  with  the  fatigue 
of  the  nerve,  and  with  the  residual  nervous  stimulation. 
But  closely  connected  with  them  are  the  "contrast" 
colours  that  are  seen  on  a  gray  background  after  the 
eye  has  been  fatigued  by  looking  at  any  coloured  object. 
The  tints  of  these  "contrast"  colours  are  approxi- 
mately, though  not  accurately,  the  complementaries  of 
the  respective  colours  that  have  excited  them.  Thus 

after  staring  intently  for 
some  time  at  a  bright 
green  disk,  the  after-image 
against  a  'gray  wall  is  of 
a  reddish  tint.  There  are 
many  ways  of  showing 
these  tints.  I  will  give 
you  as  an  example  that 
used  here  in  this  theatre 
by  the  late  Professor 
Tyndall.  Against  the 
white  wall,  half -lit  with 
daylight,  I  hold  up  on 
the  end  of  a  stick  a 
cardboard  disk  about  a  foot  in  diameter  covered 
with  bright  blue  paper  (Fig.  56).  The  beams  of  an 
electric  lamp  are  directed  upon  it  to  make  its  tint  more 
brilliant.  You  must  look  at  it  fixedly  while  I  count 
thirty  in  a  distinct  voice.  When  I  come  to  "thirty"  I 
will  drop  the  disk ;  but  you  must  continue  to  look  at 
the  same  region  of  the  wall,  where  you  will  see — now 
that  I  drop  the  disk, — a  yellow  image  or  ghost,  of  the 
same  size  as  the  blue  disk.  In  like  manner  if  I  hold  up 


FIG.  56. 


VISIBLE  SPECTRUM  AND  THE  EYE 


93 


a  red  disk  you  will,  when  your  eye  is  fatigued,  see  a  green 
or  peacock  coloured  ghost.  The  reason  of  the  contrast 
tint  is  that  if  you  have  fatigued  the  eye,  or  any  region 
of  the  retina  of  the  eye,  with  red  waves,  that  region  will 
be  less  sensitive  for  red  than  it  is  for  the  other  colours. 
Hence  if  gray  (i.e.  diluted  white)  is  presented  in  view, 
the  retina  at  the  fatigued  region  is  more  sensitive  to 
all  the  other  tints  present  than  it  is  to  red,  and  will 
therefore  on  the  whole  receive  an  impression  in  which 
green  predominates. 

Another  way  to  see  the  contrast  tints  is  to  stretch 
upon  a  ring  of  cardboard  a  sheet  of  semi-transparent 
coloured  tissue  paper,  and  then 
upon  this  as  a  background  gum 
a  smaller  ring  of  white  card- 
board. Diffuse  daylight  should 
be  allowed  to  fall  from  the  front 
upon  the  white  card,  making  it 
gray.  By  lights  suitably  placed 
behind  the  coloured  tissue  is 
lit  up.  The  eye  therefore  sees 
the  gray  ring  between  an  inner 
and  an  outer  circle  of  colour. 

And  after  looking  for  a  very  few  seconds  will  pronounce 
the  gray  card  to  have  become  of  a  complementary  tint. 
Thus  if  the  tissue  paper  is  orange  in  hue  the  gray  card 
ring  takes  a  bluish-peacock'  colour  by  contrast. 

But  the  persistence  of  impressions  in  the  eye  is  not 
limited  to  phenomena  of  colour.  All  ordinary  visual 
impressions  last  a  perceptible  time,  the  images  of  brightly 
lighted  objects,  even  when  only  viewed  for  a  thousandth 


FIG 


•  57- 


94  LIGHT  LECT. 

of  a  second  will  take  a  whole  tenth  of  a  second  to  die 
away.     If,  then,  you  can  present  to  the  eye  a  second 


FIG.  58 

impression  before  the  first  one  has  died  away,  the  effect 
is  the  same  as  though  both  had  been  present  at  one 
time.  A  familiar  toy  depending  on  this  principle,  and 
called  the  thaumatrope,  consists  of  a  bit  of  white  card 


VISIBLE  SPECTRUM  AND  THE  EYE 


95 


held  between  two  strings  on  which  it  can  be  twirled. 
On  one  side  there  is  painted  say  a  horse ;  on  the  other 
his  rider.  On  blowing  against  the  card  to  twirl  it  you 
seethe  rider  (Fig.  58)  mounted  on  his  horse.  We  may 
try  this  experiment  in  a  new  Way.  On  one  side  of  a 
vertical  card  is  painted  in  outline  a  birdcage.  On  the 


other  side,  a  bird.  By  a  b^.id  and  pulley  we  spin  the 
card  rapidly ;  and  lo  !  you  see  the  bird  within  its  cage. 

I  hold  in  the  lant  .n  a  small  disk  of  sheet  metal 
having  a  number  of  holes  pierced  in  it ;  giving  on  the 
screen  a  lot  of  bright  points  of  light.  Then  on  making 
this  disk  vibrate  on  the  end  of  a  spring,  or  rotate  about 
a  pin,  each  little  white  hole  is  transformed  apparently 
into  a  luminous  line,  moving  about  on  the  screen. 

Another  optical  illusion  depending  chiefly  upon  the 
persistence  of  vision  is  afforded  by  the  strobic  circles 
which  I  devised  in  1877.  On  giving  these  black  and 


96 


LIGHT 


LECT. 

white  patterns  a  small  "  rinsing  "  motion,  the  circles  and 
toothed  wheels  seem  to  rotate  on  their  axis. 

The  latest  of  optical  illusions  and  one  not  easy  to 
explain,1  is  Benham's  colour-top.  A  number  of  narrow 
black-lines  are  drawn  as  arcs  of  circles  of  various  lengths, 
upon  a  white  surface,  half  of  which  (Fig.  61)  is  coloured 


FIG.  60. 


FIG.  61 


black.  On  revolving  this  disk,  and  viewing  it  by  a 
sufficiently  strong  light,  the  arcs  of  some  of  the  circles 
appear  coloured.  The  rotation  must  be  neither  too 
slow  nor  too  quick.  On  reversing  the  rotation  the  order 
of  the  colours  reverses.  The  effect  appears  to  be  due 
to  the  intermittent  stimulation. 

1  See  recent  paper  in  Proceedings  of  the  Royal  Society,  by  Mr. 
Shelford  Bidwell,  F.R.S.,  whose  explanation  is  that  when  the 
particular  nerve- fibres  which  give  the  red  sensation  are  excited  at 
any  part  of  the  retina,  the  immediately  adjacent  parts  of  the  same 
nerve-fibres  are  for  a  short  period  sympathetical!y  affected,  so  that 
a  red  border  seems  for  an  instant  to  grow  around  the  image  of  a 
white  object  suddenly  seen.  In  the  same  way,  when  the  image  of 
a  white  object  is  suddenly  cut  off  there  is  a  sympathetic  reaction 
giving  a  transient  blue  border  around  the  disappearing  image. 


ii  VISIBLE  SPECTRUM  AND  THE  EYE  97 

Another  example  of  effects  produced  by  persistence  of 
the  optical  impressions  in  the  eye  is  afforded  by  an  old 
toy,  the  zoetrope,  or  wheel  of  life;  in  which  the.  sem- 
blance of  motion  is  given  to  pictures  by  causing  the  eye 
to  catch  sight,  in  rapid  sequence,  through  moving  slits,  of 
a  series  of  designs  in  which  each  differs  slightly  from  the 
one  preceding.  Thus  if  you  want  to  make  the  sails  of 
a  windmill  seem  to  go  round,  the  successive  pictures 
must  represent  the  sails  as  having  turned  round  a  little 
during  the  brief  moment  that  elapses  between  each 
picture  being  glimpsed  and  the  next  being  seen.  These 
intervals  must  be  less  than  a  tenth  of  a  second,  so  that 
the  successive  images  may  blend  properly,  and  that  the 
movement  between  each  picture  and  the  next  may  be 
small.  Mr.  Muybridge  has  very  cleverly  applied  this 
method  to  the  study  of  the  movements  of  animals. 
Anschiitz's  moving  pictures,  illuminated  by  intermittent 
sparks,  were  the  next  improvement.  And  the  latest 
triumph  in  this  development  of  the  subject  has.  been 
reached  in  the  animatogmph,  which  the  inventor,  Mr. 
R.  Paul,  has  kindly  consented  to  exhibit. 

The  animatograph  pictures  are  photographed  upon 
a  travelling  ribbon  of  transparent  celluloid ;  the  time 
which  elapses  between  each  picture  being  taken  and 
the  next  being  about  one -fiftieth  of  a  second.  A 
scene  lasting  half  a  minute  will,  therefore,  be  repre- 
sented by  about  1500  pictures,  all  succeeding  one 
another  on  a  long  ribbon.  If  these  pictures  are  then 
passed  in  their  proper  order  through  a  special  lantern, 
with  mechanism  that  will  bring  each  picture  up  to  the 
proper  place  between  the  lenses,  hold  it  there  an 

H 


98 


LIGHT 


LECT. 


FIG.  62. 


instant,  then  snatch  it  away  and  put 
the  next  in  its  place,  and  so  forth, 
the  photograph  projected  on  the 
screen  will  seem  to  move.  You  see 
in  a  street  scene,  for  example,  the 
carts  and  omnibuses  going  along; 
the  horses  lift  their  feet,  the  wheels 
roll  round,  foct  passengers  and 
policemen  walk  by.  Everything 
goes  on  exactly  as  it  did  in  the 
actual  street.  Or  you  see  some 
children  toddling  beside  a  garden 
seat.  A  big  dog  comes  up,  and  the 
boy  jumps  astride  of  him,  but  falls 
off  (Fig.  62),  and  rises  rubbing  his 
bumps.  Or  a  passenger  steamer 
starts  from  Dover  pier :  you  see  her 
paddles  revolve,  the  crowd  on  the 
pier  wave  farewells  with  handker- 
chiefs or  hats,  the  steamer  wheels 
round,  you  see  the  splash  of  foam, 
you  note  the  rolling  clouds  of  black 
smoke  proceeding  from  her  funnel, 
then  she  goes  out  of  sight  round 
the  corner.  The  reality  of  the 
motions  is  so  great  that  you  feel  as 
though  you  had  veritably  seen  it  all 
with  your  own  eyes.  And  so  you 
have.  You  have  just  as  truly  seen 
the  movements  of  the  scene  as  when 
you  have  listened  to  the  phonograph 


ii  VISIBLE  SPECTRUM  AND  THE  EYE  99 

you  have  heard  the  voice  which  once  impressed  the 
record  of  its  vibrations.  Of  all  the  animatograph 
pictures  those  that  appeal  most  to  me  are  the  natural 
scenes,  such  as  the  waves  rolling  up  into  a  sea-cave 
and  breaking  on  the  rocks  at  its  mouth,  and  dashing 
foam  and  spray  far  up  into  its  interior.  Nothing  is 
wanting  to  complete  the  illusion,  save  the  reverberat- 
ing roar  of  the  waves. 

Note. — Since  the  delivery  of  these  lectures,  Mr.  Shelford  Bidwell, 
F.R.S.,  has  pursued  the  subject  of  the  curious  colour  -  effects 
mentioned  at  the  foot  of  p.  96,  and  has  reached  some  extraordinary 
results.  A  cardboard  disk  8  inches  in  diameter  is  half-covered  with 
black  velvet,  the  other  half  being  left  white  or  gray.  A  sector  of 
45°  is  cut  away  at  the  junction  of  the  black  and  white  portions,  and 
this  disk,  suitably  balanced,  is  mounted  upon  a  revolving  apparatus 
to  rotate  with  a  speed  of  about  6  to  8  turns  per  second.  Behind 
it,  so  as  to  be  visible  at  each  turn  through  the  gap  where  the 
sector  has  been  cut  away,  is  placed  a  coloured  picture,  and  a  bright 
lamp  is  placed  a  few  inches  in  front  to  illuminate  it.  The  direction 
of  rotation  is  such  that  the  open  sector  is  preceded  by  black  and 
followed  by  white.  On  thus  viewing  a  picture  by  intermittent  vision, 
each  part  appears  of  a  pale  colour  complementary  to  its  actual 
tint.  A  red  rose  with  green  leaves  appears  a  green  rose  with 
reddish  leaves.  A  blue  star  on  a  yellow  ground  appears  as  a  yellow 
star  on  a  bluish  ground.  Black  printing  on  a  white  paper  appears 
whitish  printing  on  a  grayish  paper,  and  so  forth. 


APPENDIX  TO  LECTURE   II 

ANOMALOUS    REFRACTION    AND    DISPERSION 

ON  p.  78  attention  is  drawn  to  the  circumstance  that  the 
spectrum  as  produced  by  a  prism  is  irrational  ;  that  is  to 
say,  that  the  dispersion  is  such  that  the  different  waves  are 
not  spaced  out  in  proportion  to  their  wave-length,  the  red 
and  orange  waves  .being  relatively  crowded  together  at  one 
end  of  the  spectrum,  while  the  violet  and  blue  waves  are 
unduly  spread  out.  But  the  dispersion  is  different  on 
different  substances.  In  fact,  no  two  substances  disperse 
the  light  in  exactly  the  same  way,  though  in  general  the 
order  of  the  colours  is  the  same,  and  the  general  trend  of 
the  irrationality  is  to  compress  the  red  end.  But  there  are 
a  few  known  substances  in  which  this  irrationality  becomes 
excessive,  and  develops  into  an  entirely  abnormal  dispersion 
in  which  the  violet  waves  are  less  refracted  than  the  red  ! 
This  phenomenon  of  anomalous  dispersion  was  first  noticed  l 
in  1840  by  Fox  Talbot  in  some  crystals  of  the  double 
oxalate  of  chromium  and  potassium.  The  colours  of  the 
spectra  of  some  of  these  crystals  were  so  anomalous  that  he 
could  only  explain  them  "  by  the  supposition  that  the 
spectrum,  after  proceeding  for  a  certain  distance,  stopped 
short  and  returned  upon  itself."  In  1861  Le  Roux  found 
that  vapour  of  iodine,  which  transmits  only  red  and  blue, 
actually  retards  the  red  more  than  the  blue,  and  gives  an 
inverted  spectrum.  Christiansen  in  1870  noticed  that  an 
alcoholic  solution  of  magenta  (rosaniline)  has  an  ordinary 
refraction  for  the  waves  from  red  to  yellow,  the  yellow  being 

1  See  Tail's  Light,  p.  156. 


APP.  REFRACTION  AND  DISPERSION  101 

refracted  more  than  orange,  and  orange  than  red,  but 
it  absorbs  green  powerfully,  and  all  the  rest  of  the  colours 
— commonly  called  more  refrangible — are  in  this  substance 
refracted  less  than  the  red  !  In  this  case,  the  spectrum 
literally  returns  back  upon  itself.  Other  observations  have 
been  added  by  Kundt  and  others.  In  particular,  Kundt 
discovered  that  some  of  the  metals,  when  made  up  into 
excessively  thin  prisms,  possess  an  anomalous  dispersion. 

The  first  point  to  note  in  discussing  this  phenomenon  of 
anomalous  dispersion  is  that  it  only  occurs  in  highly 
coloured  substances.  It  is  closely  related  to  the  circum- 
stance that  in  these  substances  there  is,  by  reason  of  their 
molecular  constitution,  a  strong  absorption  for  waves  of 
some  particular  wave-length.  Thus  in  rosaniline,  which 
has  a  strong  absorption-band  in  the  green,  the  fact  that 
green  light  is  absorbed  appears  to  exercise  a  perturbing 
influence  upon  the  waves  of  the  shorter  kinds,  causing  them 
to  be  less  refracted  instead  of  more.  These  remarkable 
phenomena  obviously  have  something  to  do  with  the  way 
in  which  the  molecules  of  ponderable  matter  are  con- 
nected with  the  ether.  None  of  the  dispersion  formulae  of 
Cauchy,  Ketteler  or  others  gave  a  satisfactory  account  of 
them. 

In  1872  Lord  Rayleigh  considered  the  problem  of  the 
refraction  of  light  by  opaque  bodies,  and  in  the  Philosophical 
Magazine  (vol.  xliii.  p.  322)  gave  the  following  exceedingly 
suggestive  comment : — 

"  On  either  side  of  an  absorption-band  there  is  an 
abnormal  change  in  the  refrangibility  (as  determined  by 
prismatic  deviation)  of  such  a  kind  that  the  refraction  is 
increased  below  (that  is  on  the  red  side  of)  the  band,  and 
diminished  above  it.  An  analogy  may  be  traced  here  with 
the  repulsion  between  two  periods  which  frequently  occurs 
in  vibrating  systems.  The  effect  of  a  pendulum,  suspended 
from  a  body  subject  to  horizontal  vibration,  is  to  increase  or 
diminish  the  virtual  inertia  of  the  mass  according  as  the 
natural  period  of  the  pendulum  is  shorter  or  longer  than 
that  of  its  point  of  suspension.  This  may  be  expressed  by 
saying  that  if  the  point  of  support  tends  to  vibrate  more 


102  LIGHT  LECT.  II 

rapidly  than  the  pendulum  it  is  made  to  go  faster  still,  and 
vice  versa.  Below  the  absorption-band  the  material  vibra- 
tion is  naturally  the  higher,  and  hence  the  effect  of  the 
associated  matter  is  to  increase  (abnormally)  the  virtual 
inertia  of  the  ether  and  therefore  the  refrangibility.  On 
the  other  side  the  effect  is  the  reverse." 

In  1893  von  Helmholtz  published  a  remarkable  study1 
based  on  Maxwell's  electromagnetic  theory  of  light.  The 
essence  of  this  theory  is  as  follows  : — 

The  electromagnetic  waves  passing  through  the  ether 
travel  at  a  rate  which  is  retarded  by  the  presence  of 
material  molecules,  the  ether  being  as  it  were  loaded  by 
them.  These  heavy  particles  cannot  be  set  into  vibration 
without  taking  up  energy  from  the  advancing  wave  ;  and  so 
long  as  there  is  no  absorption,  they  give  up  this  kinetic 
energy  again  to  the  wave  as  it  passes  on.  In  this  way  the 
velocity  of  propagation  of  the  train  of  waves  is  slightly  less 
'than  the  velocity  of  propagation  of  the  individual  wave  ; 
the  front  wave  of  the  train  continually  dying  out  in  giving 
its  energy  to  the  material  particles  in  the  medium.  In  such 
a  medium  there  will  of  course  be  ordinary  refraction  ;  and 
as  the  velocity  of  propagation  of  the  wave-train  will  depend 
on  the  frequency  (i.e.  on  the  wave-length)  of  the  oscillations 
(there  being  in  general  a  greater  retardation  of  the  waves  of 
higher  frequency,  i.e.  of  shorter  wave-length),  there  will  be  a 
dispersion  of  the  ordinary  kind.  All  this  applies  to  waves 
the  wave-length  of  which  is  large  compared  with  the  size 
of  the  molecules.  But  if  there  were  smaller  waves,  the 
frequency  of  which  coincided  very  nearly  with  the  natural 
oscillation-period  of  the  molecules  or  atoms,  such  waves 
would  set  up  a  violent  sympathetic  vibration  of  these 
material  particles,  and  would  be  strongly  absorbed. 
•Suppose  that  there  are  waves  of  still  smaller  size  and  still 
•higher  frequency.  Their  oscillations  are  too  rapid  to 
affect  the  atoms  ;  they  pass  freely  between  the  interstices  of 

1  See  Wudemands  Annalen,  xlviii.  p.  389.  The  fullest  account 
of  this  that  has  appeared  in  English  is  in  The  Electrician,  xxxvii. 
p.  404,  and  an  abstract  account  by  Professor  Oliver  Lodge,  ib. 
p.  37i~(JuTy  1896). 


APP.  REFRACTION  AND  DISPERSION  103 

matter  and  are  not  retarded,  therefore  not  refracted  or  dis- 
persed, or  only  very  slightly  so.  The  medium  would  act  as 
if  almost  perfectly  transparent  to  such  waves  ;  and  their 
refraction  might  be  either  slightly  negative  or  slightly 
positive  ;  whilst  for  the  minutest  waves  of  all  the  refraction 
would  be  simply  zero.  The  formula  which  von  Helmholtz 
deduced  is  in  its  simplest  form  the  following  : — 


,2-."2 


where  /x,  is  the  refractive  index  of  the  medium  (supposed 
quite  transparent),  n  the  frequency,  and  a  and  ft  constants 
depending  on  the  material.  To  interpret  the  formula,  con- 
sider what  it  reduces  to  in  the  following  cases  (i)  n  much 
smaller  than  a  or  ft ;  (2)  n  —  ft  ;  (3)  ;/  =  a;  and  (4)  n 
much  greater  than  a  or  /3.  In  the  first  case,  n  being  small 
we  are  dealing  with  long,  slow -period  waves  such  as 
Hertzian  waves  or  those  of  infra-red  dimensions.  Neglect- 
ing rfi  compared  with  a2  or  ft2  the  formula  reduces  to  [j. 
=  a//?,  being  independent  of  wave-length.  In  the  second 
case  if  the  frequency  is  such  that  n  =  ft  the  medium  cannot 
possibly  be  transparent,  as  there  would  be  violet  absorp- 
tion. The  real  meaning  is  that  as  n  increases  from  case 
( i )  toward  the  value  n  =  ft  the  refractive  index  increases, 
and  would  become  indefinitely  great  were  it  not  for  the 
absorption  that  sets  in.  In  the  state  of  things  between 
cases  (2)  and  (3),  where  n  is  larger  than  ft  but  smaller  than 
a,  the  values  of  yu,  calculated  by  the  formula  are  imaginary  ; 
but  owing  to  the  absorption  they  would  in  reality  diminish 
clown  to  near  zero,  that  is  to  say,  the  refraction  in  these 
conditions  becomes  negative.  This  corresponds  to  the  state 
of  things  observed  by  Kundt  with  their  refracting  prisms 
of  iron,  nickel,  and  platinum,  which  refract  the  light  toward 
the  refracting  edge  instead  of  from  it.  As  n  increases  from 
case  (3)  when  it  equals  a,  the  zero  value  of  /x  gradually 
changes,  and,  when  n  becomes  very  great  compared  with 
a  and  j3,  it  approaches  to  unity,  so  that  for  excessively 
short  waves  there  is  no  refraction  at  all. 


104 


LIGHT 


LECT.   II 


Consider  the  particular  value  of  n  for  which  /x  becomes 
a  maximum.  This  is  the  case  in  which  the  excessive 
absorption  makes  the  medium  practically  opaque.  For 
values  of  n  a  little  less  than  this  there  will  be  practically 
complete  transparency  and  ordinary  refraction  and  dis- 
persion ;  for  values  of  n  a  little  greater  than  this  there  will 
again  be  practical  transparency,  but  there  will  be  a  refraction 
in  the  wrong  direction  (/x  being  less  than  unity),  and  the 
dispersion  will  be  anomalous.  Fig.  63  illustrates  this 
dependence  of  //,  upon  n.  In  the  case  of  rosaniline,  the 
frequency  for  which  the  absorption  becomes  excessive  is 
about  578  billions  per  second,  corresponding  to  a  wave-length 


*  * 

t*» 

c 

4> 
»- 

,1 

^ 

fc£ 

^ 

c 
o 

1  5* 

\  o 

.0.5 
i| 

7=  ? 

I 

»  ^ 

*! 

^—  — 

^ 

7 

I 

Line 

Ofter 

flff 

\ 

7 

Frequency 

FIG.  63. 

of  21  millionths  of  an  inch  in  air.  E.  F.  Nichols  has  found 
that  quartz  shows  a  similar  change  of  properties  for  infra-red 
waves  of  a  frequency  between  36  and  45-4  billions  per 
second.  For  almost  all  ordinary  transparent  substances 
the  absorption-band  occurs  a  long  way  down  in  the  ultra- 
violet ;  in  some  it  may  possibly  occur  in  the  infra-red.  It 
may  be  possible,  for  example,  for  flint-glass,  the  refractive 
index  of  which  for  ordinary  light  is  between  1-5  and  17,  to 
have  a  refractive  index  as  high  as  2-6  for  disturbances  of 
very  low  frequency  such  as  Hertzian  waves  :  that  being  the 
theoretical  value  for  long  waves  as  required  by  Maxwell's 
theory  to  correspond  to  the  square-root  of  the  observed 
dielectric  constant.  Probably  many  substances  have  more 
than  one  absorption-band,  thus  still  further  complicating  the 
anomalous  dispersion. 


LECTURE   III 

POLARISATION    OF    LIGHT 

Meaning  of  polarisation — How  to  polarise  waves  of  light  —  Illus- 
trative models — Polarisers  made  of  glass,  of  calc-spar,  and  of 
slices  of  tourmaline — How  any  polariser  will  cut  off  polarised 
light — Properties  of  crystals — Use  of  polarised  light  to  detect 
false  gems — Rubies,  sapphires,  and  amethysts — Polarisation  by 
double  refraction — Curious  coloured  effects,  in  polarised  light, 
produced  by  colourless  slices  of  thin  crystals  when  placed 
between  polariser  and  analyser  —  Further  study  of  comple- 
mentary and  supplementary  tints  —  Exhibition  of  slides  by 
polarised  light — Effects  produced  on  glass  by  compression,  and 
by  heating. 

SCIENTIFIC  men  often  fall  into  the  habit  of  using  long 
and  difficult  words  to  express  very  simple  and  easy 
ideas.  The  natural  consequence  is,  that  people  are 
often  led  to  think  that  there  is  something  difficult  about 
a  really  easy  subject,  whereas  the  main  difficulty  is  to 
understand  the  meaning  of  the  words  selected  to  de- 
scribe it. 

The  word  "polarisation,"  used  in  optics,  is  one  of 
these  terms.  It  sounds  very  learned  and  difficult,  but 
the  idea  it  is  intended  to  express  is  really  very  simple. 
Let  me  try  to  give  you  the  idea  before  we  try  to  fit  any 
name  to  it. 


io6  LIGHT  LECT. 

In  my  first  lecture  I  endeavoured  to  give  you  some 
simple  notions  about  waves  and  the  way  they  travel.  I 
asked  you  particularly  to  distinguish  between  the  oscil- 
latory motions  of  the  particles  and  the  forward  travelling 
of  the  waves  themselves.  Let  us  return  to  the  motions 
of  the  particles.  Suppose  any  particle  or  group  of  par- 
ticles to  have  motion  given  to  it,  a  rapid  "  to-and-fro  "- 
in  other  words,  let  it  be  supposed  to  vibrate.  Then  if 
it  is  surrounded  by  a  suitable  medium,  and  its  vibra- 
tions occur  with  a  sufficiently  great  frequency,  it  will  set 
up  waves  in  the  surrounding  medium  which  will  start 
off  from  it,  travelling  away  at  a  definite  speed  depending 
on  the  rigidity  and  density  of  the  medium.  In  the  case 
of  a  compressible  surrounding  medium  such  as  air,  the 
vibrating  body  (if  vibrating  between  the  limits  of  fre- 
quency appropriate  for  sound — that  is  to  say,  between 
about  30  per  second  and  38,000  per  second)  will  com- 
press the  air  in  front  of  itself  as  it  moves  forward,  and 
rarefy  the  air  behind  it  as  it  moves  back,  with  the 
^  result  that  it  sends  off  waves  of  condensation 
and  rarefaction.  If,  as  in  Fig.  64,  the  oscillating 

O|     body  is  a  sphere  moving  rapidly  up  and  down 

along  the  short  path  AB,  it  will  tend  alternately 

B      to  condense  and  rarefy  the  air  above  and  below 

•1G'  64'  it,  and  these  compressions  and  rarefactions  will 
travel  off  upwards  and  downwards,  spreading  a  little 
as  they  go.  But  hardly  any  waves  of  compression  and 
rarefaction  will  travel  off  sideways  from  the  oscillating 
sphere,  because  in  oscillating  up-and-down  it  does 
not  either  condense  or  rarefy  the  air  at  its  sides. 
The  wave  in  this  case  would  be  described  as  a  longi- 


A 


m  POLARISATION  OF  LIGHT  107 

tudinal  wave,  meaning  one  which  is  propagated  along 
the  line  in  which  the  particular  motion  exists — in  this 
case  vertical.  If  you  want  to  know  more  about  the 
travelling  of  sound-waves,  you  must  read  Professor 
TyndalPs  delightful  book  On  Sound ;  or  if  you  are  deep 
students  you  will  study  Lord  Rayleigh's  two  mathe- 
matical volumes  on  the  Theory  of  Sound. 

But  now,  suppose  that  you  have  to  deal  not  with  a 
medium  like  air  that  is  compressible,  but  with  a  medium 
like  jelly  that  is  incompressible,  and  in  which  the 
density  is  small  compared  with  the  rigidity  that  it 
opposes  to  any  rapid  shear.  If  in  this  case  you  set  up 
an  oscillation  with  a 
sufficiently  great  fre- 
quency, waves  will  be  /*'\  /'"\ 
set  up  which  will  travel 
off  at  a  high  rate,  but  *g 

not  in  the  line  of  the  FlG-  6s- 

motion.  On  the  contrary,  they  will  travel  off  sideways 
in  all  directions  in  ripples.  Let  Fig.  65  represent  a 
sphere  embedded  in  the  middle  of  a  great  block  of 
surrounding  jelly,  and  that  it  is  made  to  oscillate  up- 
and-down  as  before,  but  with  a  great  rapidity.  It 
cannot  move  up  without  tending  to  tear  or  shear  the 
jelly  all  around  its  girth,  nor  can  it  move  down  without 
tending  to  tear  or  shear  the  jelly  downwards ;  and 
these  shearing  stresses  travel  outward  in  all  direc- 
tions, so  that  a  particle  at  a  will,  as  these  waves  or 
ripples  in  the  solid  jelly  reach  it,  tend  also  to  move  up 
and  down.  In  any  medium,  whether  a  jelly  or  not,  if 
the  particles  are  in  such  relation  to  one  another  that 


io8  LIGHT  LECT. 

the  movement  of  any  of  them  tends  to  set  up  a  shearing 
stress,  then  that  medium  will,  like  the  jelly,  propagate 
the  disturbances  sideways.  The  waves  in  such  cases 
would  be  described  as  transverse  waves  —  meaning 
waves  which  are  propagated  in  directions  at  right  angles 
to  the  direction  in  which  the  to-and-fro  displacements 
are  executed. 

Now  the  waves  of  light  are  of  the  transverse  kind ; 
and  though  they  can  pass  through  air,  are  not  waves  of 
the  air  as  sound-waves  are.  Waves  of  light  can  cross 
the  most  perfect  vacuum ;  they  travel  thousands  of 
millions  of  miles  in  the  vacuous  space  between  the  stars. 
They  are  waves  of  another  medium  which,  so  far  as  we 
know,  exists  all  through  space,  and  which  we  call,  using 
Sir  Isaac  Newton's  term,  the  ether.  If  you  ask  me  what 
the  ether  is  made  of,  let  me  frankly  say  I  do  not  know. 
But  if  light  consists  of  waves,  and  if  those  waves  can 
travel  across  the  millions  of  miles  that  separate  the  stars 
from  the  earth,  then  it  is  clear  that  they  must  be  waves 
of  something ;  they  are  not  air- waves  nor  water-waves, 
because  interstellar  space  is  devoid  both  of  air  and  of 
water.  They  are  waves  of  a  medium  which,  though 
millions  of  times  less  dense  than  water  or  air,  has  yet  a 
property  that  resists  being  torn  or  sheared  asunder ;  ex- 
ceeding the  resistance  to  shear  even  of  hard-tempered 
steel.  Though  it  is  not  a  jelly,  since  things  can  move 
through  it  more  freely  than  you  or  I  can  move  through 
the  air,  yet  it  resembles  the  jelly  in  this  property  of 
resistance  to  shear,  and  propagates  vibrations  transversely 
to  the  direction  of  their  displacements.  Though  we 
know  neither  the  density  of  the  ether  (though  it  must 


in  POLARISATION  OF  LIGHT 


be  very  small)  nor  its  rigidity  to  shear  (which  must  be 
very  great),  we  do  know  something  which  depends  on 
the  ratio  of  these  two  properties,  namely,  the  velocity  of 
propagation  of  those  ether-waves  which  we  call  "  light  " 
(see  Appendix,  p.  156). 

Well,  now  having  got  this  notion  about  transverse 
waves,  let  us  go  back  to  the  wave-motion  model  which 
we  used  in  the  first  lecture.  It  has,  as  you  will 
remember  (Fig.  2,  p.  8),  a  row  of  little  white  particles, 
along  which  row  the  wave  is  propagated  from  left  to  right, 
though  each  little  particle  moves  up  and  down.  It  is, 
therefore,  a  model  of  a  transverse  wave  ;  the  direction  of 
travel  of  the  wave  is  at  right  angles  to  the  direction  of 
the  displacements. 

But  if  a  wave  is  to  travel  along  a  line  of  march,  from 
A  to  B,  we  may  fulfil  the  condition  of  transverse  vibra- 
tion in  other  ways.  The  small  to-and-fro  motions  must 
be  executed  across  the  line  of  march  ;  and  they  may  be 
across  the  line  of  march  without  being  vertical  —  they 
may  be  horizontal,  or  oblique.  If  I  turn  my  wave-motion 
model  on  its  side,  the  little  white  particles  now  move 
horizontally  toward  you  and  from  you,  but  the  wave  still 
travels  from  left  to  right. 

If  I  stretch  across  the  room  a  long  indiarubber  cord, 
holding  one  end  of  it  in  my  hand,  I  can  throw  it  into 
transverse  vibrations.  If  I  move  my  hand  rapidly  up- 
and-down,  I  produce  up-and-down  vibrations.  If  I 
move  my  hand  right-and-left,  I  get  right-and-left  vibra- 
tions. If  I  move  my  hand  obliquely  to-and-fro,  I  produce 
oblique  vibrations  ;  and  the  cord  transmits  them  all. 

Now,  all  that  the  word  polarisation  means  is  that  the 


1 10 


LIGHT 


LECT. 


motions  are  being  executed  in  some  particular  transverse 
direction.  If  the  vibrations  are  polarised  vertically,  that 
means  that  they  are  up-and-down  waves  that  are  travel- 
ling along.  If  I  say  that  the  light  is  polarised  hori- 
zontally, all  I  mean  is  that  the  motions  are  executed 


FIG.  66. 


right-and-left  across  the  line  of  march.     Can  anything 
be  simpler? 

Here  is  a  lump  of  jelly.  It  will  serve  excellently  to 
show  how  a  polarised  vibration  is  propagated.  I  stick 
into  it  horizontally  two  pins  with  silvered  heads — one  at 

one  side,  the  other  at  the 
other.  If  I  give  a  sudden 
displacement  to  one  pin  it 
quivers,  and  the  jelly  carries 
on  the  motion  to  the  other. 
And  note,  if  I  strike  one  so 
as  to  make  it  quiver  up-and-down,  the  other  quivers  up- 
and-down — here  we  have  a  vibration  polarised  vertically. 
If  I  make  one  quiver  right-and-left  the  other  quivers 
right-and-left — here  we  have  a  vibration  polarised  hori- 
zontally. If  I  make  one  quiver  circularly  round  and 
round,  the  other  quivers  round  and  round  also ;  giving 
an  illustration  of  a  circularly  polarised  vibration. 


FIG.  67. 


in  POLARISATION  OF  LIGHT  in 

Now  let  us  go  to  the  waves  of  light  themselves.  If 
you  look  at  a  beam  of  white  light  you  cannot  by  the 
eye l  tell  whether  it  is  polarised  to  move  up-and-down, 
or  right-and-left.  In  fact  you  cannot  tell  whether  it  is 
polarised  at  all.  Naturally,  if  the  waves  are  so  ex- 
cessively small,  and  vibrate  so  many  millions  of  millions 
of  times  a  second,  your  eye  cannot  catch  their  motions. 

The  fact  is  that  light  of  any  natural  kind,  whether 
from  the  sun,  an  electric  lamp,  a  flame,  or  any  other 
source,  is  non-polarised;  that  is  to  say,  it  consists  of 
vibrations  which  are  not  specially  directed  either  up-and- 
down  or  right-and-left,  or  in  any  other  one  direction. 
Natural  light,  given  out  by  hot  bodies,  is  absolutely 
miscellaneous.  Not  only  does  it  consist,  as  we  saw  in 
the  last  lecture,  of  a  lot  of  different  colours — that  is,  of 
different  wave-lengths,  mixed  up  together — but  it  con- 
sists of  waves  whose  direction  of  transverse  vibrations 
are  also  all  jumbled  up.  At  one  instant  they  may  be  up- 
and-down;  then  they  change  to  right-and-left,  or  to 
oblique,  or  circular,  or  elliptical,  or  possibly  to  some- 
thing still  more  complex.  Just  think  how  the  light 
starts  from  the  white-hot  tip  of  the  carbon  pencil  in 
my  electric  lamp.  The  particles  of  white-hot  carbon 
are  in  fierce  vibration,  jostling  against  one  another,  and 
in  jostling  impart  vibrations  to  the  ether — setting  up 

1  Not  as  thrown  on  the  screen.  But  the  eye  can  be  trained  to 
detect  the  plane  of  polarisation,  for  example,  of  light  from  the  blue 
sky,  which  is  naturally  polarised  in  directions  at  right  angles  to  the 
position  of  the  sun.  The  training  consists  in  being  able  to  recog- 
nise certain  appearances  called  "  Haidinger's  brushes"  which  result 
from  the  feeble  polarising  properties  of  the  refracting  structures  of 
the  eye. 


112  LIGHT  LECT. 

ether-waves.  When  any  one  particle  gets  a  sudden  jolt 
it  quivers,  and  gives  out  a  vibration,  which  we  may 

represent  by  the  curve  (Fig. 
68),  with  a  lot  of  little  wave- 
lets each  like  its  fellow,  per- 
haps several  thousands1  of 
them  before  they  die  away. 
Each  such  vibration  would 

die  away  like  the  note  of  a  piano -string  struck  and 
left  to  itself.  But  perhaps  before  the  motion  has  died 
away  another  jolt  sets  it  off  vibrating  in  a  new  direc- 
tion, again  to  die  away.  Suppose  millions  of  these 
little  particles,  all  jostling,  and  vibrating,  and  sending 
out  trains  of  wavelets.  It  is  clear  that  one  ought  to 
expect  the  utmost  admixture  of  wave-sizes  and  directions 
of  vibration  in  the  resultant  light. 

Then,  you  understand,  that  as  natural  light  is  not 
polarised  in  any  particular  direction,  if  we  want  to  get 
polarised  light  we  must  do  something  to  it  to  polarise  it. 
But  how  ? 

1  According  to  the  researches  of  Fizeau,  at  least  50,000,  on  the 
average,  in  ordinary  light.  Prof.  Michelson's  more  recent  experi- 
ments, in  which  he  has  obtained  interference  between  two  waves 
the  paths  of  which  differed  by  more  than  20  cm.  or  1,000,000  wave- 
lengths, prove  that  the  average  number  of  wavelets  in  each  train 
must  be  reckoned  in  millions. 


[TABLE 


Ill 


POLARISATION  OF  LIGHT 


TABLE  III. — POLARISERS 


Principle. 

Nature  of  Apparatus. 

Reference. 

By  Reflexion  . 

I. 
II. 

Black  glass  at  about  57° 
Delezenne's  Polariser  . 

(P-  153). 

(P-  I23)- 

By  Refraction          .      -| 

III. 
IV. 

Glass  sheet  at  about  57° 
Bundle   of    thin    glass 

(P-  154)- 

sheets  set  obliquely 

(p.  154). 

By  Double  Refraction  -I 

V. 
VI. 

Rhomb  of  Iceland  Spar 
Double-image  Prism   . 

(p.  120). 
(p.  125). 

By  Double  Refraction,) 
with  Absorption     .      / 

VII. 

Slice  of  Tourmaline     . 

(p.  119). 

By  Double  Refraction,  \ 

VTTT 

Nicol's  Prism   and  its 

with  Internal  Reflexion  / 

modern  Varieties     . 

(p.  121). 

In  Table  III.  I  have  set  down  some  eight  different 
ways  of  polarising,  which  we  will  presently  consider  in 
their  order.  But  before  we  deal  with  any  of  them,  let 
us  go  back  to  the  vibrations  of  cords  and  see  how  they 
can  be  polarised. 

Here  (Fig.  69)  is  an  indiarubber  cord  passing  through 
a  wooden  box  with  vertical  partitions.  These  partitions 
limit  the  movements  and  only  allow  vertical  vibrations 
to  pass  through.  If  I  vibrate  the  cord  in  any  way,  it  is 
only  the  vertical  components  of  the  vibration  that  suc- 
ceed in  getting  through.  The  waves,  after  passing 
through  the  box,  come  out  polarised  in  a  vertical  plane. 
If  I  turn  the  box  over  on  its  side  (Fig.  70)  it  will  now 
transmit  only  horizontal  components  of  vibration.  What 
will  happen,  then,  if  I  pass  the  cord  through  a  second 
box,  as  in  Fig.  70  ?  That  depends  on  the  positions  of 
the  boxes.  If  the  first  one  P  is  set  with  its  partitions 

I 


LIGHT 


LECT. 


bo 


vertical,  it  will  polarise  the  waves  vertically,  and  as  these 
waves  travel  on  they  will  come  to  the  second  box  marked 


in  POLARISATION  OF  LIGHT  115 

A.  If  this  also  has  its  partitions  vertical,  the  vertical 
waves  will  get  through  it  also.  If  both  boxes  are  turned 
over  on  their  side,  then  the  first  one  will  polarise  the 
waves  horizontally,  and  the  horizontally  polarised  waves 
will  pass  through  both  boxes.  But  if  I  have  the  first  box 
P  set  vertically  and  the  second  box  A  horizontally  (Fig. 
71),  P  will  polarise  the  vibrations  so  that  they  will  not 
get  through  A,  but  will  be  cut  off.  However  P  is 
placed  it  will  polarise  the  waves ;  if  A  is  turned  so  as 
to  cross  the  waves  they  will  be  cut  off. 

Upon  the  lecture  table  is  another  model  which  illus- 
trates the  same  set  of  facts  more  fully.  If  you  under- 
stand it  you  will  have  no  difficulty  in  understanding  the 
optical  apparatus  that  we  are  going  to  use.  In  this 
apparatus  the  vibrations  of  a  thin  silk  cord — best  seen 
by  those  in  front  of  the  table — are  produced  by  attach- 
ing one  end  to  the  prong  of  a  tuning-fork,  the  vibrations 
of  which  are  maintained  by  an  electromagnetic  attach- 
ment. To  the  distant  end  of  the  cord  is  attached  a  small 
weight,  which  has  been  so  adjusted  that  the  cord  is  thrown 
into  stationary  waves.  In  brief,  the  vibrations  of  the  cord 
are  tuned  to  those  of  the  fork.  To  polarise  the  vibrations, 
the  motions  of  the  cord  are  confined  by  means  of  a  pair 
of  glass  plates  mounted  in  wooden  cylinders  (Figs.  72, 
73).  At  the  first  nodal  point  of  the  cord  the  first  pair 
of  glass  plates  acts  as  a  polariser,  P ;  the  cord  beyond 
that  point  vibrating  in  the  plane  thus  imposed  upon  it. 
A  pointer  fixed  upon  the  wooden  cylinder  shows  the 
direction  of  the  plane  of  polarisation.1  The  second 

1  Concerning  the  term,  "plane  of  polarisation,"  see  remarks  in 
Appendix  to  this  Lecture,  p.  158. 


LIGHT 


LECT. 


pair  of  glass  plates  is  set  at  the  second  nodal   point 
to  act  as  an  analyser^  A.     The  vibrations  of  the  cord 


in  POLARISATION  OF  LIGHT  117 

are  made  vertical  by  the  polariser  P,  and  when  the 
plane  of  the  analyser  A  is  also  vertical  (as  in  Fig.  72) 
the  vibrations  which  pass  through  the  polariser  pass 
through  the  analyser  also.  But,  if  (as  in  the  previous 
experiment  with  the  boxes)  the  analyser  is  turned  round 
a  quarter,  so  that  the  slit  between  the  glass  plates  lies 
across  the  vibrations  (as  in  Fig.  73)  the  vibrations  are 
no  longer  transmitted.  To  recapitulate,  the  vibrations 
are  transmitted  when  the  polariser  and  analyser  are 
parallel  to  one  another :  but  are  cut  off  and  extinguished 
when  polariser  and  analyser  are  crossed.  Hence,  by 
turning  round  the  analyser  to  such  a  position  that  it 
cuts  off  the  vibrations  we  can  ascertain  with  accuracy * 
the  direction  of  the  vibrations  proceeding  from  the 
polariser. 

But  why  should  we  linger  longer  upon  mere  models 
when  we  can  operate  with  light -waves  themselves? 
My  assistant  throws  upon  the  screen  a  beam  of  white 
light  from  the  electric  lamp  within  the  optical  lantern. 
He  now  places  in  the  path  of  the  beam  a  large  polariser, 
P  (Fig.  74).  What  this  polariser  is,  I  will  presently 
explain.  He  now  S2ts  it  so  that  it  polarises  the  light, 
allowing  to  fall  upon  the  screen  those  waves  only  whose 
vibrations  are  executed  in  a  vertical  plane.  The  white 
disk  of  light  on  the  screen  consists,  in  fact,  of  up-and- 
down  light  only.  Your  eye  would  not  tell  you  whether 
the  light  was  vibrating  up  and  down,  or  even  that  it  was 

1  The  model  will  enable  the  orientation  of  the  plane  of  the  vibra- 
tions to  be  determined  to  within  about  half  a  degree  of  angle.  That 
is,  if  the  analyser  is  as  much  as  half  a  degree  out  of  the  crossed 
position,  the  vibrations  are  not  completely  extinguished. 


LIGHT 


LECT. 


polarised  at  all.  To  ascertain  that  the  waves  are  really 
polarised  we  must  have  recourse  to  an  analyser.  This 
analyser,  A,  is  itself  simply  a  smaller  polariser.  In 
order  that  you  may  see  it  the  better  it  is  mounted 
(see  Fig.  75)  by  thin  strings  upon  a  ring  -  support, 
the  shadow  of  which  you  see  on  the  screen.  If  this 
is  also  set  in  the  proper  position  to  transmit  up-and- 
down  vibrations,  the  polarised  light  will  come  through 


Lens 


FIG. 


it,  both  polariser  and  analyser  being  clear  as  glass.  If 
now  the  analyser  A  is  turned  round  one  quarter  it  will, 
though  clear  as  glass,  entirely  cut  off  the  up-and-down 
vibrations,  with  the  result  (Fig.  76)  that  no  light  gets 
through  it.  This  cutting  off  of  the  light  by  turning 
'the  analyser  one  quarter  round  proves  that  the  light  was 
polarised.  When  the  planes  of  polariser  and  analyser 
are  parallel  to  one  another — both  vertical,  or  both 
horizontal, — then  we  have  the  "bright  field"  of  trans- 
mitted light.  When  the  planes  of  polariser  and  analyser 


POLARISATION  OF  LIGHT 


119 


are    crossed — one  vertical,  the    other   horizontal — then 
the  light  is  cut  off,  and  we  have  the  "dark  field." 

There  is  a  gem  called  the  tourmaline  which,  when 
cut  into  thin  slices,  has  the  property  of  polarising  light. 
This  gem l  is  often  found  of  a  dark  green  colour,  but 
also  of  brown,  dark  blue,  and  even  ruby  tint.  Into  the 
beam  of  ordinary  white  light  now  cast  upon  the  screen 


FIG.  75. 


FIG.  76. 


there  is  now  introduced  a  thin  slice  of  brown  tourmaline 
(Fig.  77).  It  looks  dark,  for  it  cuts  off  more  than  half 
the  light.  But  such  light  as  succeeds  in  getting  through 
is  polarised — the  vibrations  being  parallel  to  the  longer 
dimension  of  the  slice.  A  second  thin  slice  of  tourmaline 
is  now  introduced,  and  superposed  over  the  first.  When 
they  are  parallel  to  one  another  light  comes  through 
both  of  them  (Fig.  78).  But  if  one  of  them  is  now 

1  The  dark  green  tourmaline  is  also  sometimes  called  the  Brazilian 
emerald,  though  it  is  of  entirely  different  composition  from  an 
emerald.  The  bishops  of  the  South  American  Catholic  churches 
wear  tourmalines  in  their  episcopal  rings,  instead  of  emeralds. 


120 


LIGHT 


LECT. 


turned  round,  so  that  they  are  crossed,  as  in  Fig.  79,  no 
light  can  get  through  the  crossed  crystals.  The  one  cuts 
off  all  horizontal  vibrations  and  horizontal  components 
of  vibration,  the  other  cuts  off  all  vertical  vibrations  and 
vertical  components  of  vibration.  Hence,  when  crossed, 
they. produce  a  "dark  field."  One  acts  as  polariser,  the 
other  as  analyser. 

Let  us  return  to  the  big  polariser  (Fig.  74)  which  we 
used  in  the  previous  experiment,  and  which  was  as  clear 
as  glass.  It  is  made  of  Iceland  spar,  a  natural  crystal, 


FIG. 


77- 


FIG.  78. 


FIG.  79. 


which  once  was  common  but  now  is  rare  and  expensive. 
As  imported  from  the  mine  in  Iceland  this  spar  possesses 
the  peculiar  property  known  as  "double  refraction": 
when  you  look  through  it  you  see  everything  double. 
Here  is  a  fine  specimen  mounted  in  a  tube.  Look  at 
your  finger  through  it ;  you  will  see  two  fingers.  It  is  a 
substance  which  splits  the  waves  of  light  into  two  parts, 
giving  two  images ;  and,  moreover,  polarises  the  light  in 
the  act  of  splitting  it,  so  that  each  part  is  polarised. 
*We  do  not,  however,  want  both  images ;  we  want  only 
one.  What  do  we  do  ?  We  adopt  the  method  proposed 
eighty  years  ago  by  William  Nicol,  a  celebrated  Scotch 


Ill 


POLARISATION  OF  LIGHT 


121 


philosopher,  and  construct  out  of  a  crystal  of  the  spar 
a  "polarising  prism,"  or  Nicol  prism.     Here  are  several 


Direction 

>•>» >/-/ 

of  Light 


F.G.  80. 


Nicol  prisms  of  various  sizes ;  and  also  several  modern 
modifications l  of  the  Nicol  prism.  Here  also  is  a  large 
wooden  model  to  illustrate  Nicol's  method. 

1  In  Foucault's  modification,  a  film  of  air  is  interposed  between 
the  two  wedges  of  crystal.  In  Hartnack's  prism  a  film  of  linseed 
oil  is  interposed,  and  the  ends  of  the  wedges  are  squared  off.  I  have 
myself  from  time  to  time  suggested  several  modifications  which  are 


FIG. 


FIG.  82. 


improvements  upon  the  original  Nicol  prism.  In  one  of  these,  the 
natural  end- faces  of  the  prism  are  sliced  away  parallel  to  the 
crystallographic  axis  so  as  to  leave  terminal  faces  that  are  "  principal 
planes"  (Fig.  81),  and  the  crystal  is  then  sliced  with  an  oblique  cut 


122  LIGHT  LECT. 

Selecting  a  piece  of  Iceland  spar  of  suitable  propor- 
tions we  slice  it  across  (with  a  piece  of  copper  wire, 
used  as  a  saw,  and  some  emery  powder)  in  an  oblique 
direction  from  one  of  its  two  blunt  corners  to  the  other ; 
polish  the  surfaces,  thus  dividing  the  prism  into  two 
wedges.  These  are  then  cemented  together  again 
with  Canada  balsam  (a  resinous  cement);  and  the 
polarising  prism  is  complete.  Its  operation  upon  light 
is  as  follows.  When  the  waves  enter  through  one  end- 
face  they  are  split  into  two  parts  which  take  slightly 
different  directions,  and  strike  at  different  angles  upon 
the  film  of  balsam.  As  a  consequence  one  of  the  two 
beams  when  it  meets  the  film  of  balsam  is  reflected 
off  sideways,  as  from  an  oblique  mirror,  while  the 
other  goes  through  the  prism  and  emerges  at  the  other 
end-face.  Consequently  only  one  of  the  two  beams 
gets  through  the  prism,  the  other  being  suppressed  or 
reflected  out  of  the  way.  Prisms  made  in  Nicol's  way 

that  is  also  a  principal  plane,  and  these  wedges  are  then  reunited 
with  Canada  balsam  or  linseed  oil.  In  a 
cheaper  modification — a  "reversed  Nicol" 
— the  natural  end-faces  are  cut  off  (Fig. 
82)  so  as  to  reverse  the  shape,  and  the 
oblique  cut  is  then  made  along  a  re- 
versed diagonal  and  is  nearly  in  a 
"principal  plane."  In  a  third  modifica- 
tion the  end-faces  are  first  trimmed  off 
obliquely  as  principal  planes  of  section 
through  one  of  the  natural  edges  of  the 
end  -  face ;  an  oblique  cut  is  then  given 
(as  in  Fig.  83)  between  two  of  the 

terminal  arretes,  from  FM   to  GN,  and  the  two   pieces  are  then 

transposed  ;  and  they  are  finally  reunited  by  balsam  along  two  of 

their  natural  faces. 


Ill 


POLARISATION  OF  LIGHT 


123 


FIG.  84. 


have  usually  oblique  end-faces  of  diamond  shape.     The 

vibrations   which    pass    through    are    those 

executed   in   the  direction    parallel    to    the 

shorter    diagonal    (Fig.    84);    while    those 

which  are  suppressed  are  those  parallel  to 

the   longer  diagonal.      The   large    polariser 

used  in  front  of  the  lantern  (Fig.  74,  p.  118)  is  simply 

a  large  Nicol  prism.1 

1  In  consequence  of  the  dearth  of  spar,  large  Nicol  prisms  can 
only  be  procured  at  extravagant  prices.  In  1888  Mr.  Ahrens  con- 
structed for  me  a  large  reflecting  polariser,  having  a  clear  aperture 
of  2§  inches.  For  projection  purposes  it  is  quite  equal  to  a  Nicol 
prism  of  equal  aperture,  and  is  much  less  costly.  In  this  reflecting 
polariser,  which  is  constructed  on  a  principle  suggested  by  Delezenne, 
the  light  is  first  turned  to  the  proper  polarising  angle  (about  57°)  by 
a  large  total-reflexion  prism  of  glass  cut  to  a  special  shape.  It  is 
then  reflected  back  parallel  to  its  original  path  by  impinging  upon 
a  mirror  of  black  glass  covered  by  a  single  sheet  of  the  thinnest 

patent  plate  glass  to  increase 
the  intensity  of  the  light. 
Fig.  85  shows  the  design  of 
this  prism.  Compared  with 
a  large  Nicol  prism  it  has 
one  disadvantage  :  it  cannot 
be  conveniently  rotated,  so 
that  it  polarises  the  light  in 
a  fixed  plane.  To  obviate 
this  defect,  I  devised  an 
"optical  rotator"  to  place 
on  the  end  of  the  prism. 
This  consists  simply  of  two 
plates,  qi  and  q^  of ' '  quarter- 
wave "  mica  ;  the  first  of 
them  being  mounted  with  its  axis  fixed  at  45°  to  the  plane  of  polar- 
isation ;  the  second  q^  being  mounted  in  a  revolving  frame  .which 
can  be  turned  to  any  desired  position.  The  effect  of  rotating  this 
plate  is  to  rotate  the  plane  of  polarisation. 


124 


LIGHT 


LECT. 


Let  us  devote  a  little  further  attention  to  this  pheno- 
menon of  the  double-refraction  that  thus  yields  us  two 
beams  of  light  that  are  polarised  in  different  planes. 
Here  is  another  wave-motion  model  (Fig.  86)  constructed 
to  show  two  sets  of  waves  which  are  polarised  in  planes 
mutually  at  right  angles  to  one  another.  Here  are  two 


FIG.  86. 

waves  of  silvered  beads,  both  01  which,  when  I  turn  the 
handle  of  the  model,  will  march  along.  Both  have  the 
same  wave-length,  both  march  at  the  same  pace  and 
toward  the  same  end.  But  there  is  this  difference 
between  them :  in  one  the  displacements  are  polarised 
at  45°  one  way ;  in  the  other  the  displacements  are  at 
45°  the  other  way.  Of  course  there  is  some  mechanism 
inside  the  box  to  make  them  move  thus ;  but  they  illus- 


in  POLARISATION  OF  LIGHT  125 

trate  what  is  meant  by  saying  that  there  can  be  two 
waves  polarised  at  right  angles  to  one  another. 

But  the  point  still  remains  why  should  the  spar  so 
act  on  the  light  as  to  split  it  into  two  oppositely  polarised 
beams?  Let  us  first  prove  that  the  spar  really  does 
this.  Here  is  a  piece  of  spar  mounted  as  a  double- 
image  prism.1  In  front  of  the  lantern  is  placed  first  a 
metal  diaphragm  having  a  round  hole  in  it,  the  image  of 
which  is  focused  on  the  screen,  giving  a  circular  white 
spot.  Now  interposing  the  double-image  prism  we  see 
that  it  splits  the  light  into  two  parts,  diverting  half  the 
light  away  from  the  original  spot, 
and  producing  a  second  one  which 
(with  -this  size  of  aperture  in  the 
diaphragm)  overlaps  the  first.  On 
rotating  the  double-image  prism 
it  is  seen  that  the  ordinary  image 
remains  stationary,  whilst  the  ex- 
traordinary image  revolves  around 
it.  Now  to  prove  that  they  are 

each  polarised,  I  interpose  a  Nicol  prism.  On  rotating 
it,  it  is  observed  to  cut  off  first  one  of  the  two  spots  and 
then  the  other.  If  the  Nicol  prism  is  set  so  as  to 

1  That  is  a  prism  of  spar  mounted  in  such  a  way  as  to  throw 
both  the  images  upon  the  screen.  There  are  several  modes  in 
which  such  may  be  constructed.  The  usual  mode  is  to  take  a 
wedge  of  spar  cemented  to  a  corresponding  wedge  of  glass.  The 
"  extraordinary  "  beam  (that  is  to  say,  the  beam  whose  vibrations 
are  executed  in  planes  parallel  to  the  crystallographic  axis)  goes 
straight  through  the  "  ordinary  "  beam,  and  emerges  obliquely, 
giving  a  displaced  image.  If  the  prism  is  made  of  two  wedges  of 
spar  cut  at  different  angles  and  crossed,  the  "  ordinary "  image  is 
the  central  one  while  the  "extraordinary"  image  revolves. 


126 


LIGHT 


LECT. 


transmit  only  vertical  vibrations,  while  the  double-image 
prism  is  rotated  it  is  observed  that  when,  as  in  Fig. 
88,  the  two  images  are  in  the  position  vertically  above 
one  another,  the  ordinary  is  cut  off,  while  the  extra- 
ordinary is  transmitted.  On  turning  round  the  double- 
image  prism  the  ordinary  image  gradually  appears,  while 
the  extraordinary  fades  away,  until  when  the  prism  has 
been  turned  one  quarter  round  the  ordinary  image  is 
transmitted,  and  the  extraordinary  cut  off,  as  is  evident 
from  Fig.  89.  If  the  prism  is  turned  to  45°  both  images 


FIG.  88. 


FIG.  89. 


FIG.  90. 


are  equally  bright,  the  directions  of  the  resolved  vibra- 
tions being  as  shown  by  the  fine  lines  in  Fig.  90. 

Now,  having  proved  that  the  spar  does  split  the 
light  into  two  beams  in  which  the  vibrations  are 
moving  in  different  ways,  we  have  yet  to  consider  how 
it  effects  this.  The  resolution  of  oblique  movements 
into  two  components  at  right-angles  to  one  another  is  an 
important  principle  in  mechanics,  and  one  which  is  best 
illustrated  for  our  purpose  by  means  of  a  model.  Sup- 
pose that  there  is  a  displacement  taking  place  obliquely, 
it  can  always  be  resolved  into  two  parts — a  part  which 
is  up-and-down,  and  a  part  which  is  right-and-left  in 


Ill 


POLARISATION  OF  LIGHT 


127 


direction.  The  model  (Fig.  91)  is  fixed  upon  a  board, 
on  the  corner  of  which  is  drawn  a  little  diagram.  Sup- 
pose the  oblique  motion  to  be  from  A  to  B,  then  you  can 
resolve  that  oblique  motion  into  two  parts — a  vertical 
part  marked  AV,  and  a  horizontal  part  marked  AH. 
Every  schoolboy  knows  the  problem  called  the  "  paral- 
lelogram of  forces,"  according  to  which  when  two  forces 
act  on  one  point  at  the  same  time,  they  combine  into  a 


FIG.  91. 


single  oblique  resultant  along  the  diagonal  of  the  paral- 
lelogram of  which  the  two  component  forces  are  sides. 
Well  our  present  problem  is  simply  the  converse  to  that. 
Here  in  the  model  are  two  wooden  slides,  with  cross- 
heads.  One  can  slide  up-and-down  only  :  the  other 
right-and-left  only.  Running  in  grooves  in  the  two 
cross-heads  is  a  roller  fixed  to  the  end  of  a  third  bar  of 
wood,  which  can  be  set  in  any  oblique  position,  and 
then  slid  along  obliquely  by  hand.  If  I  set  this  third 
bar  horizontal  and  slide  it  along,  all  the  movement  it 


128  LIGHT  IECT. 

gives  is  horizontal — there  is  no  vertical  component.  If 
I  set  it  vertical  and  slide  it  up  and  down,  it  simply  moves 
the  vertical  slide  up  and  down.  But  if  I  set  it  obliquely, 
and  slide  it  along,  then  part  of  its  motion  produces  a 
movement  of  the  vertical  slide,  while  the  horizontal 
component  of  its  motion  produces  a  movement  of  the 
horizontal  slide.  How  much  of  the  motion  will  be 
vertical,  and  how  much  horizontal,  will  depend  obviously 
on  the  angle  at  which  the  original  motion  is  imparted. 
Now  there  is  one  particular  angle  at  which  the  resolved 
portions  would  be  equal  to  one  another.  What  is  that 
angle  ?  If  I  set  the  bar  which  produces  the  displace- 
ment exactly  midway,  at  45°,  that  displacement  will  be 
resolved  into  two  equal  components.  You  will  remember 
that  at  45°  the  double-image  prism  yielded  two  images  of 
equal  brightness. 

Such  a  resolution  into  two  parts  can  be  produced  on 
oblique  waves  of  light  by  any  of  the  crystals  here,  such 
as  Iceland  spar,  quartz,  mica,  or  selenite,  provided  the 
crystal  possesses  a  particular  kind  of  structure.  The 
condition  is  that  the  crystal  shall  possess  a  greater  optical 
rigidity — a  greater  stiffness  that  is — in  one  direction 
than  in  another.  You  know  what  one  means  when  one 
talks  about  the  grain  in  a  piece  of  wood — how  much 
easier  it  is  to  split  in  one  direction  than  in  another. 
But  this  grain,  which  depends  on  the  fibrous  structure, 
manifests  itself  in  other  ways  than  by  ease  of  cleavage. 
A  piece  of  wood  is.  harder  to  bend  along  the  grain  than 
across  it.  And  so  with  some  crystals :  they  possess  an 
invisible  structure,  a  grain  so  fine  that  we  cannot  see 
the  fibres  or  lines  of  structure ;  but  that  grain  manifests 


in  POLARISATION  OF  LIGHT  129 

itself  in  various  ways.  There  are  differences  in  ease  of 
cleavage,  and  also  differences  in  rigidity1  in  different 
directions.  This  particular  crystal — Iceland  spar — has 
a  greater  rigidity  in  one  direction  than  in  another,  and, 
as  a  result,  any  wave  of  light  passing  obliquely  into  it 
is  split  into  two  portions,  one  having  vibrations  parallel 
to  the  axis  of  greatest  rigidity,  and  another  portion 
having  vibrations  parallel  to  the  axis  of  least  rigidity  ; 
therefore  at  right  angles  to  the  former.  That  is  why  it 
splits  the  light  into  two  parts,  and  why  those  parts  are 
each  polarised.  Some  crystals,  namely,  diamonds  and 
garnets,  are  equally  rigid  in  all  directions,  and  therefore 

1  The  word  rigidity  is  here  preferred,  though  in  many  treatises 
it  would  be  spoken  of  as  "elasticity."  To  say  that  the  crystal 
possesses  different  "  elasticities "  in  different  directions — which 
is  quite  correct — would  convey  to  many  people  an  erroneous  idea, 
because  of  the  incorrect  way  in  which  the  word  "  elastic  "  is  often 
used.  Elastic  does  not  mean  that  a  thing  can  be  easily  stretched. 
The  hardest  hard  steel  is  more  perfectly  elastic  than  a  bit  of  india- 
rubber,  but  it  certainly  is  not  so  stretchable.  In  saying  that  it  has 
elasticity  we  mean  that  however  little  we  may  succeed  in  compress- 
ing or  stretching  it,  it  returns  back,  when  released,  to  its  former 
shape  or  size.  In  scientific  treatises  that  substance  is  regarded  as 
having  the  highest  co-efficient  of  elasticity  which  requires  the 
greatest  stress  to  produce  a  given  deformation  or  strain.  When  we 
are  dealing,  as  in  the  case  of  transverse  displacements,  with  motions 
tending  to  shear  (see  p.  107  above)  the  medium,  the  particular 
elasticity  to  be  considered  is  the  elasticity  which  resists  shearing, 
and  for  this  the  term  rigidity  is  entirely  appropriate.  In  all  this 
optical  work  we  are  of  course  dealing  not  with  the  mechanical 
elasticity,  but  with  the  optical  elasticity,  that  is  to  say,  with  the 
elasticity  of  the  ether  within  the  substance.  It  is  this  which  in  the 
crystals  in  question  is  regarded  as  greater  in  one  direction  than 
in  another.  The  greater  the  rigidity  the  higher  is  the  velocity  of 
propagation  of  the  wave  whose  displacements  are  in  that  direction. 
See  Appendix  to  Lecture  III.  p.  156. 

K 


130  LIGHT  LECT. 

these  do  not  show  any  double  refraction,  nor  do  they 
polarise  the  light. 

Here,  again,  is  a  model  to  illustrate  the  splitting  of 
the  waves.  Two  thin  flexible  strips  of  ebonite,  O  and  E 
(Fig.  92),  are  inserted  into  saw-cuts  in  the  end  of  a 
cylinder  of  wood.  In  the  other  end  I  can  fix  a  third 
strip,  A.  Notice,  if  you  please,  that  O  is  set  with  its 
edge  vertical,  and  is  capable  of  vibrating  right  and  left ; 
while  E  is  set  with  its  edge  horizontal,  and  can  vibrate 
up  and  down  only.  Holding  the  wooden  cylinder  in 
my  left  hand,  I  apply  my  right  hand  to  give  a  vibratory 


FIG.  92. 


movement  to  the  strip  A  at  the  other  end.  If  I  waggle 
A  from  right  and  left,  then  O  vibrates  from  right  and  left. 
If  I  waggle  A  up  and  down,  then  E  vibrates  up  and 
down,  while  O  is  quiet.  If  now  I  impart  to  A  an 
oblique l  motion,  both  O  and  E  vibrate,  the  oblique 
motion  being  resolved  into  a  vertical  part  and  a  hori- 
zontal part. 

Returning  to  the  waves  of  light  themselves,  let  us 
"now  see  some  of  the  beautiful  effects  which  result  from 
these  operations  of  splitting  the  vibrations  into  two 
parts,  of  recompounding  them  after  passing,  through  a 

1  The  end  of  the  cylinder  into  which  A  is  fixed  is  capable  of 
being  turned  round  on  a  joint  at  J,  the  cylinder  being  made  in  two 
parts  fitting  on  one  another. 


in  POLARISATION  OF  LIGHT  131 

slice  of  crystal,  and  then  of  analysing — that  is  to  say, 
resolving — the  light  that  falls  upon  the  screen. 

In  front  of  the  lantern  there  has  been  set  the  large 
Nicol  prism  as  polariser,  and  in  front  of  that  a  smaller 
Nicol  prism  as  analyser.  When  the  latter  is  turned  to 
cross  the  former  we  have  the  dark  field  (p.  1 1 9),  all  light 
being  cut  off.  The  only  light  that  passes  through  the 
first  Nicol  consists  of  vertical  vibrations,  and  as  the 
second  Nicol  is  set  to  transmit  horizontal  vibrations  only, 
nothing  comes  through  it.  Now  I  take  up  a  piece  of 
thin  mica  (that  crystalline  substance  of  which  lamp- 
shades are  sometimes  made,  and  often  miscalled  "talc"), 
and  hold  it  obliquely  in  the  path  of  the  polarised  light 
on  its  way  from  the  polariser  to  the  analyser — in  fact, 
between  the  two  Nicols.  See  how  it  brings  light  into 
the  dark  field.  In  Tyndall's  expressive  phrase  it  seems 
to  "scrape  away  the  darkness."  See,  too,  what  beauti- 
ful tints  it  shows !  Yet  it  itself  is  perfectly  colourless. 
Why,  then,  this  light  and  these  colours?  Well,  the 
light  is  easily  explained.  The  crystal  possesses  a 
greater  rigidity  in  one  particular  direction  through 
its  substance  —  an  "'axis  of  maximum  elasticity,"  as 
it  is  called.  If  we  set  the  slice  with  that  axis 
obliquely  (Fig.  93)  across  the  direction  of  the  vertical 
vibrations,  then  it  will  resolve  those  vertical  vibrations 
into  two  parts,  one  part  parallel  to  the  axis  of  maxi- 
mum elasticity  and  the  other  at  right  angles  to  that 
axis.  These  two  sets  of  waves  in  the  crystal  will  both 
be  oblique.  They  travel  through  the  thickness  of  the 
slice  at  unequal  speeds,  and  when  they  emerge  again  at 
the  other  side  of  the  crystal  one  set  of  vibrations  will 


132 


LIGHT 


LECT. 


have  got  a  little  behind  the  other.  Hence  the  two 
components  as  they  emerge  and  recombine  will  not 
produce,  as  their  resultant,  vibrations  that  are  vertical. 
The  resultant  vibrations  may  be  oblique,  or  even 
elliptical;  their  precise  nature  and  orientation  will 
depend  on  the  nature  and  thickness  of  the  slice  of 
crystal  used,  and  upon  the  wave-length  of  the  light. 
But  the  immediate  point  is  that  the  resultant  emerging 


ANALYSER 


FIG.  93. 


waves  will  no  longer  be  polarised  vertically  ;  the  crystal 
slice  will  have  so  split  up,  retarded,  and  recombined 
the  vibrations  that  they  emerge  vibrating  in  other  direc- 
tions. Hence  this  emerging  light  when  it  falls  upon  the 
analyser  will  possess  some  horizontal  components,  and 
these  will,  as  you  see,  be  transmitted.  The  colours 
depend  upon  the  thickness  of  the  slice  of  crystal ; 
which  in  this  case  is  very  irregular,  seeing  that  it  is 
merely  a  rough  piece  split  off  with  a  pocket  knife. 


in  POLARISATION  OF  LIGHT  133 

This  is  mica ;  but  there  is  another  kind  of  thin  crystal 
known  as  selenite  (or  gypsum),  which  is  also  readily 
split  with  the  knife,  and  produces  similar  effects.  There 
are  plenty  of  other  crystals  that  will  produce  similar 
effects. 

Here,  mounted  in  a  small  glass  cell,  are  two  rubies. 
Putting  them  in  front  of  the  polariser,  and  adjusting  a 
lens  to  focus  them  on  the  screen,  you  see  how  much 
alike  they  are.  One  is  a  genuine  ruby,  though  slightly 
flawed ;  the  other  is  a  sham  ruby  of  glass,  also  slightly 
flawed.  Which  is  which  ?  You  may  guess  or  choose  ; 
but  I  wish  to  know.  That  knowledge  we  can  obtain 
by  putting  in  front  of  the  lens  the  second  Nicol  as 
analyser.  Turning  it  to  position  we  obtain  the  dark 
field ;  and  in  that  field  one  of  the  two  gems  shines  out 
while  the  other  is  dark.-  The  one  which  shines  out  is 
the  genuine  ruby,  for  it  alone  possesses  the  axis  of 
maximum  elasticity.  If  we  slowly  revolve  the  cell  so 
as  to  make  the  images  of  the  two  gems  move  around 
one  another,  the  one  simply  remains  dark,  while  the 
other  goes  through  alternations  of  light  and  darkness. 
It  is  dark  in  those  positions  in  which  its  axis  of  maxi- 
mum elasticity  stands  either  vertical  or  horizontal,  and 
shines  brightest  in  the  two  positions  where  that  axis 
stands  at  45°  of  obliquity  to  right  or  to  left,  at  which 
angle  the  resolution  into  the  two  oblique  components  is 
most  complete. 

Now  we  place  in  our  apparatus,  between  polariser 
and  analyser,  another  cell  containing  several  assorted 
gems — a  ruby,  garnet,  topaz,  emerald,  sapphire,  chryso- 
beryl,  and  a  little  diamond  in  the  centre.  Adjusting 


134  LIGHT  LECT. 

the  analyser  to  give  us  the  dark  field,  and  then  slowly 
rotating  the  cell,  note  how  each  crystal,  when  its  axis 
of  maximum  elasticity  comes  to  the  oblique  position  of 
45°,  shines  out.  But  there  are  two  that  show  nothing — 
the  diamond  and  the  garnet,  crystals  belonging  to  the 
"  cubic  "  system,  whose  elasticities  in  all  directions  are 
equal. 

Here  are  a  few  more  objects  for  our  polariscope, 
slices  of  crystals  and  minerals.  First  of  all  a  bit  of 
amethyst.  Though  by  ordinary  light  it  is  quite  clear 
and  of  a  pale  purple  tint,  yet  when  examined  by  polar- 
ised light  it  is  at  once  evident  that  the  gem  consists  of 
a  number  of  superposed  separate  layers,  which  show 
alternately  dark  purple  and  white ;  while  some  regions 
of  the  crystal  show  strange  masses  of  unexpected  colour, 
and  these,  if  one  turns  the  analyser  round,  change  tint. 
A  second  piece  of  amethyst,  from  Brazil,  shows  a  more 
perfect  structure. 

Here  is  a  thin  slice  of  gray  Scotch  granite.  Its 
natural  mottlings  are  merely  black  and  white,  being 
composed,  as  mineralogists  will  tell  you,  of  small  crystals 
of  transparent  quartz  and  transparent  felspar,  mingled 
with  specks  of  black  mica.  But  when  viewed  by 
polarised  light  the  whole  slice  shows  wonderful  gleams 
of  colour,  and  reveals  new  details  of  structure. 

This  next  beautiful  object  is  a  thin  transverse  slice 
of  a  stalactite,  one  of  those  natural  deposits  of  calcare- 
ous spar  which  hang  like  icicles  from  the  roofs  of  caves. 
Its  deposit,  layer  by  layer,  almost  concentrically,  is 
evident;  but  in  the  polariscope  it  shows  a  mysterious 
black  cross,  which,  when  the  analyser  is  rotated,  changes 


in  POLARISATION  OF  LIGHT  135 

to  a  white  cross.  This  black  cross  is  visible  in  the  dark 
field.  Such  black  crosses  one  obtains  whenever  the 
object  is  one  having  different  rigidities  in  the  radial 
and  tangential  directions.  The  next  slide  shows  it  even 
better.  This  is  an  artificial  crystal  of  a  stuff  called 
salicine,  which  can  be  dissolved  in  alcohol.  When  the 
solution  is  poured  upon  a  warm  piece  of  glass  the 
alcohol  evaporates,  and  the  salicine  crystallises.  The 


FIG.  94. 

operation  of  crystallising  starts  at  a  number  of  inde- 
pendent centres,  around  which  little  groups  of  crystals 
grow  with  a  radial  structure  and  a  circular  outline.  As 
their  axes  of  maximum  elasticity  all  point  radially  to 
the  centre  around  which  they  grew,  the  maximum 
resolution  of  the  wave  light  will  occur  in  each  little 
circle  at  45°  to  right  and  to  left,  while  in  the  two  direc- 
tions, vertical  and  horizontal,  there  will  be  no  resolu- 
tion of  the  polarised  light,  and  these  (in  the  dark  field) 


136  LIGHT  LECT. 

will  therefore  remain  dark,  giving  the  black  crosses. 
On  revolving  the  analyser  quickly,  so  that  the  black 
crosses  change  to  white  crosses  and  then  turn  black 
again  in  rapid  succession,  all  the  little  crosses  in  the 
separate  groups  of  crystals  appear  to  revolve  like  so 
many  little  windmills. 

Here  is  a  thin  slice  of  selenite,  split  off  quite  irregu- 
larly, and  of  different  thicknesses  in  different  parts. 
Without  the  analyser  it  shows  on  the  screen  nothing 
•worthy  of  attention,  being  nearly  clear  as  glass  and 
quite  colourless.  But  replacing  the  analyser  to  produce 
the  dark  field,  at  once  a  gorgeous  set  of  tints  is  pro- 
duced. At  one  part  the  thickness  is  such  that  the  tint 
is  a  fine  orange;  just  above  it  where  the  crystal  is  a 
shade  thicker  comes  a  patch  of  brilliant  crimson.  These 
being  the  tints  in  the  dark  field,  see  how,  when  the 
analyser  is  rotated  a  quarter  so  as  to  give  the  bright 
field,  each  tint  changes  to  its  complementary,  the 
orange  turning  to  azure  blue,  and  the  crimson  turning 
to  vivid  green. 

Now  I  have  yet  to  explain  how  these  colours  come 
about.  All  I  have  said  is  that  they  depend  upon  the 
thickness  of  the  crystal  film,  and  upon  the  wave-lengths 
of  the  different  kinds  of  light.  If  on  its  passage  through 
the  crystal  the  vibrations  are  split  up  into  two  parts 
that  travel  with  unequal  speeds,  one  set  of  vibrations 
will  gain  on  the  other ;  the  one  that  is  more  retarded 
will,  when  it  emerges,  have  lost  step  with  the  other  set. 
This  "  difference  of  phase  "  due  to  the  different  speed  of 
travelling  may  in  a  thin  slice  of  crystal  be  as  little  as 
one  quarter  or  one  half  only  of  a  wave-length;  or  it 


in  POLARISATION  OF  LIGHT  137 

may,  if  the  crystal  is  thicker,  be  more  than  a  whole 
wave-length,  or  it  may  be  several  wave-lengths.  The 
question  as  to  how  the  two  sets  of  waves  will  recombine 
when  they  emerge  at  the  other  face  of  the  crystal  slice, 
will  depend  upon  the  question  how  much  one  wave  has 
got  out  of  step  with  the  other  wave.  Clearly  that  will 
depend  on  the  kind  of  light  and  on  the  thickness  of  the 
slice.  If,  for  example,  a  slice  of  mica  ^hr  inch  thick 
caused  the  waves  of  yellow  light  to  get  exactly  one 
quarter  of  a  wave-length  out  of  step  with  one  another, 
then  it  is  clear  that  a  slice  twice  as  thick  would  produce 
twice  as  much  retardation  of  phase,  and  make  the  two 
sets  of  yellow  waves  get  exactly  half  a  wave-length  out 
of  step.  Further,  if  the  thickness  were  such  as  to  make 
yellow  waves  get  exactly  half  a  wave  out  of  step,  it 
would  not  produce  an  exact  half-wave  retardation  upon 
the  larger  red  waves,  or  upon  the  shorter  violet  waves. 
The  consequence  of  all  this  is  that  in  the  recombina- 
tions of  the  emerging  waves  after  passing  through  any 
given  thickness  of  material,  the  angle  at  which  the 
vibrations  recombine  is  different  for  different  wave- 
lengths. If,  for  example,  the  crystal  thickness  is  such 
that  green  waves  in  recombining  come  out  vibrating 
nearly  vertically,  then  for  that  thickness  of  crystal  green 
light  will  be  almost  entirely  cut  off  in  the  dark  field,  but 
almost  entirely  transmitted  in  the  bright  field.  '  So  we 
have  this  complementary  relation  between  the  tints  in 
the  two  positions  of  the  analyser. 

The  succession  of  tints  for  a  regularly  increasing 
thickness  of  crystal  follows  the  order  of  the  tints  known 
as  Newton's  "Colours  of  Thin  Plates."  Sir  Isaac 


138 


LIGHT 


LECT. 


Newton    examined   the   colours   of   soap-bubbles   and 
other  thin  films,  and  ascertained  their  relation  to  the 


FIG.  95. 


thickness  of  the  film  by  ingeniously  producing  a  film  of 
air  between  two  pieces  of  glass,  one  flat,  the  other  very 
slightly  curved.  There  is  now  projected  upon  the 
screen  the  system  of  coloured  rings  ("Newton's  rings") 


in  POLARISATION  OF  LIGHT  139 

so  produced.  Where  the  two  pieces  of  glass  touch 
there  is  a  central  dark  spot ;  around  this  centre  there 
are  coloured  rings  of  light  of  the  several  "orders." 
The  tints  are  set  forth  in  the  accompanying  Table. 
Those  of  the  first  order,  beginning  with  black,  are  very 
dull,  and  end  with  a  dark  purple  or  "transition  tint," 
after  which  the  colours  of  the  second  order  follow 
almost  those  of  the  spectrum,  except  that  there  is  no 
good  green.  At  the  end  of  the  second  order  comes 
again  a  purple  "transition  tint,"  after  which  the  third 
order  gives  a  sort  of  spectrum  series  with  a  good  green 
but  with  a  poor  yellow.  To  the  third  order  succeeds  a 
fourth  with  paler  tints,  mostly  green  and  red,  then  a 
still  paler  fifth,  followed  by  higher  orders  that  are  still 
less  distinct.  The  corresponding  thicknesses  of  the  film 
are  given  in  millionth  parts  of  an  inch.  For  producing 
the  kindred  set  of  Newton's  tints  by  means  of  thin  films 
of  selenite  in  the  dark  field  of  the  polariscope  much 
greater  thicknesses  must  be  used,  because  the  pheno- 
menon is  due  to  the  difference  between  the  two  velocities 
of  the  two  sets  of  vibrations.  Thus  to  produce  a  re- 
tardation of  J  wave,  and  therefore  give  the  same  whitish 
tint  as  an  air-film  5-5  millionths  of  an  inch  thick,  a  piece 
of  selenite  must  be  used  the  thickness  of  which  is  about 
TTRRT  inch.  The  tints  so  produced — see  p.  145 — are 
not  precisely  identical  with  the  air  film  tints  because  of 
the  modifying  effect  of  dispersion  in  the  selenite. 


[TABLE 


TABLE  IV 


TINTS  OF  NEWTON'S  COLOURS  OF  THIN  FILMS. 

Order. 

Film  Thickness. 

Tint  in  Reflected  Light. 

0 

Black. 

3'5 

Gray. 

5  '5 

Whitish. 

I. 

8 

Straw. 

10 

Orange. 

10-5 

Brick  Red. 

ii 

Dark  Purple. 

"'5 

Violet. 

13 

Blue. 

15 

Peacock. 

II. 

18 

Yellow. 

I9'5 

Orange. 

21 

Red. 

22 

Violet. 

24 

Blue. 

25  '5 

Peacock. 

27 

Green. 

III. 

29'5 

Yellowish  Green. 

3i 

Rose. 

32-5 

Crimson. 

33 

Purple. 

34  '5 

Violet. 

36 

Peacock. 

38 

Green. 

40 

Yellowish  Green. 

44 

Rose. 

f 

48 

Pale  Green. 

v. 

52 

Pale  Rose. 

I 

55 

Rose. 

V,.        { 

60 
64 
66 

Pale  Peacock. 
Pale  Rose. 
Rose. 

VII.         { 

71 

74 

Pale  Qreen. 
Pale  Rose. 

LECT.  in  POLARISATION  OF  LIGHT  141 

Now  the  reason  for  this  peculiar  succession  of  tints 
arises  from  the  overlapping  of  the  successive  orders  for 
the  waves  of  different  colours.  When  produced  as 
Newton  made  them,  by  the  interference  of  light  by  re- 
flexion from  the  upper  and  lower  surfaces  of  a  film  of  air, 
there  would  be  found,  at  a  particular  distance  from  the 
centre  a  particular  thickness  of  film  such  that  the  light 
reflected  at  the  second  surface  is  exactly  half  a  wave 
out  of  step  with  the  light  reflected  at  the  first  surface. 
At  this  place — the  air  film  being  here  of  a  thickness 
equal  to  a  quarter  of  the  wave-length  of  that  kind  of 
light — that  particular  kind  of  light  would  be  cut  off  by 
self -interference.  For  example,  yellow  light  having 
waves  22  millionths  of  an  inch  long,  would  be  cut  off 
by  interference  when  the  film  is  5  J  millionths  of  an  inch 
thick.  But  as  all  the  waves  have,  to  begin  with,  lost 
half  a  wave-length  (as  evidenced  by  the  central  spot 
being  black),  by  reason  of  the  second  reflexion  being  an 
external  one,  the  result  is  that  all  round  the  centre,  at 
such  a  distance  that  the  film  is  5!  millionths  of  an  inch 
thick,  yellow  light  is  reinforced,  and  there  is  seen  a  bright 
ring — the  first  order  for  yellow  light.  As  red  waves  are 
27  millionths  of  an  inch  long  the  ring  for  the  first  order 
for  red  light  will  be  at  a  place  where  the  film  is  about 
6|  millionths  of  an  inch  thick.  Newton's  rings  then 
will  seem  of  different  sizes  in  different  kinds  of  light ; 
and  since  white  light  consists  of  all  different  colours 
mixed  up,  the  Newton  tints  will  be  produced  by  the 
overlappings  of  all  the  different  tints.  This  may  be  made 
plainer  by  considering  a  diagram  (Fig.  96)  in  which  the 
various  sizes  of  waves  are  represented  to  scale.  Let  the 


142 


LIGHT 


LECT. 


distances  measured  horizontally  from  the  left  side  repre- 
sent the  distances  from  the  centre  of  the  system  of 
Newton's  rings,  the  air-gap  being  supposed  to  widen  in 


RED 


ORANGE 


YELLOW 


GREEN 


PEACOCK       . 


BLUE  . 


VIOLET 


Resultant  Tint 


Relative 

Thickness  of 
film 


AAAAAJ 


AAAAAAAAAJ 

I  I  f  V  VI 


Millionths     of 
Inch  0 


11 


22 

FIG.  96. 


44 


proportion  to  the  radius.  Then  if  red  light  was  the 
only  kind  falling  on  the  apparatus  it  would  show  a 
system  of  red  rings  with  a  black  centre,  the  distance  of 
each  successive  red  ring  from  the  centre  corresponding 
to  the  places  where  the  crests  of  the  red  waves  occur 


in  POLARISATION  OF  LIGHT  143 

in  the  highest  row.  Similarly,  if  yellow  light  alone  were 
present,  there  would  be  a  rather  smaller  system  of 
yellow  rings  seen,  spaced  out  as  are  the  crests  of  the 
yellow  waves  in  the  third  line.  And  so  forth  for  other 
colours.  Now  since  the  rings  in  the  self-produced  light 
of  any  one  colour  are  of  a  different  size  from  the  rings 
in  the  set  produced  by  light  of  another  colour,  it  follows 
that  when  white  light  is  used,  the  sets  of  rings  of  the 
different  colours  of  which  white  light  is  compounded 
will  overlap  one  another.  At  any  given  distance  from 
the  centre  the  resultant  light  will  be  the  sum  of  all  the 
various  amounts  of  coloured  lights  at  that  distance. 
Take  the  rows  of  waves  in  Fig.  96  and  treat  them  as  if 
Fig.  96  were  an  addition  sum  of  which  we  had  to  write 
down  the  total  from  left  to  right  at  the  bottom.  At  the 
very  beginning,  on  the  left,  there  is  nothing  to  add  up, 
because  the  waves  have  not  yet  more  than  begun  to 
rise.  A  little  farther  along  all  the  waves  are  rising. 
Consider  a  distance  such  that  the  yellow  wave  is  at  its 
highest  point.  Imagine  a  vertical  line  drawn  through 
the  top  of  the  first  yellow  wave.  How  much  of  the 
other  kinds  of  light  are  present  ?  There  is  a  great  deal 
of  orange  and  some  red,  a  great  deal  of  green  and  pea- 
cock, some  blue  and  some  violet.  Now  all  these  added 
together  will  make  a  nearly  white  light,  but  rather 
yellowish  owing  to  the  preponderance  of  yellow.  The 
result  is  that  at  the  corresponding  distance  from  the 
centre  there  will  be  a  yellowish  white  ring :  and  the  air 
film  at  this  place  is  about  5^  millionths  of  an  inch  thick. 
Now  consider  a  distance  twice  as  great  from  the  centre 
of  the  rings,  or  at  the  end  of  the  first  yellow  wave,  where 


144  LIGHT  LECT. 

the  film  is  about  1 1  millionths  of  an  inch  thick.  There 
is  no  yellow,  but  there  will  be  a  very  little  orange  and 
some  red ;  there  will  be  next  to  no  green  or  peacock, 
but  there  will  be  a  little  blue  and  much  violet.  These 
colours  add  up  to  a  dark  purple  tint,  which  in  the 
coloured  diagram  on  the  screen  is  set  down  as  the  total 
in  the  bottom  line.  It  may  be  new  to  you  to  think  of 
adding  up  colours  and  putting  down  the  totals,  but  that 
is  the  way  to  reckon  out  the  resultant  tint.  Referring 
back  to  the  table  of  Newton's  tints  we  now  see  that  they 
range  themselves  in  regular  orders  with  the  purple 
transition  tint  at  the  end  of  the  first  order  where  the 
air  film  is  1 1  millionths  of  an  inch  thick,  another  purple 
tint  at  the  end  of  the  second  order  where  the  film  is  22 
millionths  thick,  and  a  third  at  the  end  of  the  third 
order,  where  the  film  is  33  millionths  thick.  In  fact 
these  darkest  tints  in  the  series  correspond  to  the 
thickness  at  which  interference  occurs  for  yellow  light. 

Now  these  Newton's  tints — produced  by  interference 
and  overlapping — are  in  general  the  same  as  the  tints 
which  result  from  the  introduction  of  our  thin  slices  of 
crystals  into  the  polariscope.  And  the  reason  why  the 
thin  slices  of  crystal  when  examined  by  polarised  light 
give  the  same  general  series  of  tints  is  as  follows. 

Suppose  the  polarised  light  to  fall  on  a  thin  slice  of 
crystal  that  has  its  axis  set  obliquely  across  the  beam. 
Then  the  vibrations  in  going  through  the  crystal  are 
split,  as  I  have  explained,  into  two  parts,  one  vibrating 
parallel  to  the  axis,  the  other  part  at  right  angles :  and 
they  do  not  take  the  same  time  to  traverse  the  crystal 
film  because  of  the  difference  between  the  rigidity  in  the 


in  POLARISATION  OF  LIGHT  145 

two  directions.  And  as  the  wave-lengths  of  different 
colours  are  different,  the  waves  of  various  colours,  though 
they  traverse  the  same  actual  thickness,  emerge  in 
different  states.  When  the  two  components  of  a  wave 
of  any  given  colour  recombine  on  emergence,  they  will 
recombine  to  form  a  vibration  in  some  new  direction, 
and  that  resultant  direction — whether  oblique  or  ellip- 
tical— will  be  different  for  different  colours.  Hence  it 
follows  that  the  analyser  will  cut  off  more  of  one  colour 
than  of  another;  and  the  light  which  comes  through 
the  analyser  will  be  the  total  of  all  the  resolved  parts 
of  each  kind  of  light.  If,  for  instance,  the  thickness  of 
the  crystal  is  such  that  the  yellow  light  on  emerging 
recombines  to  form  a  nearly  vertical  vibration,  then  the 
analyser  when  horizontal  will  cut  off  the  yellow,  and  the 
resultant  light  that  comes  through — the  total  of  all  the 
other  parts — will  be  of  a  dark  violet  hue.  Just  as  the 
colours  in  the  Newton's  rings  depend  on  the  thickness 
of  air  film  between  the  glasses,  so  do  these  colours  of 
the  film  of  crystal  in  the  polariscqpe  depend  on  the 
thickness  of  the  film  (see  p.  139). 

The  next  object  to  be  shown  you  is  a  slice  of  selenite 
that  is  exceedingly  thin  at  one  end,  and  thick  at  the 
other,  being  tapered  as  a  very  thin  wedge.  It  exhibits 
most  magnificently  the  Newton's  tints  up  to  the  end  of 
the  third  order.  It  will  serve  as  a  standard  of  com- 
parison for  other  slices.  For  instance,  there  is  now 
placed  in  the  apparatus  a  uniform  piece  of  crystal.  It 
shows  in  the  dark  field  the  red  of  the  second  order.  I 
therefore  know  that  it  is  precisely  of  the  same  thickness 
as  the  wedge  is  at  the  place  where  the  wedge  shows 

L 


146  LIGHT  LECT. 

the  same  red  tint.  This  red  turns  to  a  vivid  green 
when  the  analyser  is  rotated  so  as  to  give  the  bright 
field.  So  again  a  slice  which  in  the  dark  field  gives  a 
violet  of  the  second  order  changes  in  the  bright  field  to 
the  complementary  primrose  tint. 

I  now  take  two  prepared  pieces  of  mica,  which  will 
be  exhibited  to  you  first  separately  and  then  together. 
One  of  them  shows  the  blue  of  the  second  order,  a  tint 
which  by  reference  to  the  table  is  the  same  as  that  pro- 
duced by  an  air  film  13  millionths  of  an  inch  thick. 
The  other  shows  a  yellow  of  the  second  order,  corre- 
sponding to  an  air  film  18  millionths  of  an  inch  thick. 
Now  guess  what  will  happen  if  they  are  both  put  in  to- 
gether. Will  blue  and  yellow  make  green  ?  Not  by 
any  means.  If  superposed  (with  their  axes  both  at  45° 
to  the  right)  they  will  have  the  same  effect  as  a  piece  of 
mica  would  have  if  its  thickness  was  equal  to  that  of 
the  two  added  together  :  or  it  will  act  as  a  film  of  air  in 
the  Newton's  rings  31  millionths  of  an  inch  thick, 
giving  a  tint  which,  by  the  table,  you  see  to  be  a  rose 
red.  My  assistant  slides  one 
crystal  over  the  other  (Fig. 
97)  and  you  observe  in  this 
case  the  unexpected,  though 
predicted,  result  that  blue  and 
yellow  added  in  this  way  make 
pink.  Let  one  of  the  crystals 
now  be  turned  about  so  as  to 

put  its  axis  45°  to  the  left,  so  that  it  will  act  negatively, 
giving  the  same  result  as  if  we  had  subtracted  one  thick- 
ness from  the  other.  What  tint  ought  it  to  give  ?  Sub- 


in  POLARISATION  OF  LIGHT  147 

tracting  13  millionths  (blue)  from  18  millionths  (yellow), 
we  obtain  the  answer  that  it  ought  to  give  the  same  tint 
as  an  air  film  5  millionths  of  an  inch  thick,  which  tint  is 
a  grayish  white.  Look  for  yourselves  and  see  how  on 
the  screen  where  the  blue  (reversed)  crystal  overlaps  the 
yellow  crystal,  the  resultant  tint  is  a  grayish  white. 

The  next  object  is  a  wedge  combination  made  of 
twenty-four  very  thin  pieces  of  mica  set  to  overlap  one 
another,  so  as  to  form  a  wedge  in  steps.  It,  like  the 
smooth  wedge  of  selenite,  gives  the  Newton  tints  of  the 
first  three  orders ;  in  this  case,  however,  not  gradating 
finely  into  one  another,  but  presenting  sudden  changes 
from  the  tint  of  one  thickness  to  the  tint  of  the  next. 
Where  the  crystal  shows  the  nearest  approach  to  white, 
namely,  at  a  point  half  way  along  the  first  order,  it  cor- 
responds to  an  air  film  having  a  thickness  of  one-half  of 
the  length  of  the  wave  of  yellow  light.  Hence  such  a 
crystal  is  called  a  half-wave  plate.  If  it  is  placed  (with 
axis  also  at  45°)  upon  one  of  the  other  slices  of  crystal 
in  the  polariscope,  it  is  observed  to  raise  all  the  tints  by 
an  amount  corresponding  to  an  addition  of  1 1  millionths 
of  an  inch  to  the  corresponding  thickness  of  air  film, 
and  changes  each  to  almost  exactly 1  its  complementary 
tint.  Whitish  in  the  dark  field,  it  is  nearly  black — a 
very  dark  purple — in  the  light  field.  The  next  slide  to 
be  put  in  the  polariscope  is  an  illustration  of  this 
principle.  In  the  dark  field  you  see  a  white  swan  which, 

1  Not  precisely  exactly,  as  it  cannot  be  an  exact  half-wave  plate 
for  all  different  colours.  It  is  selected  so  as  to  be  an  exact  half- wave 
plate  for  that  tint  that  is  brightest  to  the  eye,  namely,  yellow,  or 
yellow-green. 


148  LIGHT  LECT. 

when  I  turn  the  analyser  to  give  us  the  bright  field,  changes 
to  a  black  swan.  The  space,  in  the  slide,  within  the 
outline  of  the  swan,  is  covered  with  a  piece  of  half-wave 
selenite.  I  possess  another  slide  in  which  a  baker's  boy 
attired  in  white  clothes,  with  a  sack  of  flour  on  his  back, 
changes  to  a  chimney-sweep  bearing  a  bag  of  soot. 

A  slice  of  crystal  half  as  thick  as  the  half-wave  plate 
is  called  a  quarter- wave  plate.  It  produces  a  retardation 
of  one  quarter  of  a  wave 1  (for  yellow  light)  between  the 
two  components  of  vibration  that  traverse  the  slice. 

Here  is  a  beautiful  object.  A  thin  slice  of  selenite 
has  been  ground  so  as  to  be  hollow  on  one  face  like  a 
concave  lens,  thin  in  the  middle,  thicker  at  the  edges. 
As  a  result  it  shows  Newton's  rings  in  a  far  more  splendid 
manner  than  Newton's  delicate  air  film  ever  showed 
them.  Along  with  this  concave  selenite  I  now  introduce 
a  slide  made  up  of  twelve  sectors  of  quarter-wave  crystal, 
set  with  their  axes  alternately  at  45°  to  the  right  and 
45°  to  the  left.  They  seem  to  dislocate  the  Newton's 
rings,  pushing  the  alternate  segments  in  or  out  by  one 
quarter  of  a  whole  "order"  of  tints.  Beyond  these 
objects,  and  next  the  analyser,  I  now  introduce  upright 2 

1  The  quarter- wave,  if  set  with  axis  at  45°,  to  produce  as  mentioned 
a  difference  of  phase  of  a  quarter  between  the  two  components,  pro- 
duces circularly  polarised  light.     In  all  positions  of  the  analyser 
light  still  comes  through,   nearly  equally  ;  there  is  no  dark  field. 
Also,  if  a  quarter-wave  is  placed,  along  with  other  polarising  objects 
(such  for  example  as  the  concave  selenite  next  described),  but  is  set 
with  its  axis  vertical,  instead  of  at  45°,  and  is  inserted  between  the 
polarising  object  and  the  analyser,  it  produces  great  varieties  of  tint, 
each  tint  in  changing  to  its  complementary  going,  while  the  analyser 
is  turned,  through  an  intermediate  series  of  tints. 

2  See  preceding  note. 


in  POLARISATION  OF  LIGHT  149 

a  quarter-wave  plate.  And  now,  on  rotating  the  analyser 
we  have  the  strange  appearance  of  all  these  dislocated 
rings  of  colour  marching  inwards  to  disappear  at  the 
centre,  though  succeeded  in  turn  by  other  rings.  Re- 
volving the  analyser  in  the  opposite  sense  causes  the 
rings  to  seem  to  grow  at  the  centre  and  march  outwards. 

Here  is  another  object  of  great  beauty,  a  butterfly  in 
form,  cut  out  of  selenite  ;  and  here  also  is  a  heart's-ease ; 
and  here  some  daisies  which,  though  pale  yellow  in  the 
dark  field,  turn  to  purple  Michaelmas  daisies  in  the 
bright  field.  To  pretty  devices  like  these  there  is  no 
end. 

We  may  now  apply  our  knowledge  to  a  further  study 
of  complementary  and  supplementary  tints.  A  few 
minutes  ago  I  showed  you  (Fig.  87,  p.  125)  how  the 
double-image  prism  as  analyser  gives  us  two  polarised 
images,  which,  when  the  polarised  light  passes  through 
a  circular  aperture  of  suitable  size,  overlap  one  another. 
If  with  the  same  arrangement  I  cover  the  aperture  with 
a  thin  slice  of  mica  or  selenite,  and  superpose  a  vertical 
quarter-wave  plate,  our  two  overlapping  disks  on  the  screen 
are  seen  to  give  us  two  complementary  colours,  as  though 
we  had  two  analysers,  one  of  which  had  been  turned 
through  90°.  As  we  turn  the  double-image  analyser  the 
two  images  revolve  around  one  another  exactly  as  they 
did  previously.  But  as  they  revolve  they  change  their 
colour  in  regular  successions.  And  in  every  position, 
whatever  tint  one  image  shows,  the  other  shows  the  com- 
plementary tint;  while  in  every  position  the  patch  of 
light  where  they  overlap  is  white.  But  that  is  because 
we  take  the  white  light  of  the  electric  lamp  and  are 


150  LIGHT  LECT. 

resolving  it  into  two  complementary  constituents.  Now 
if  I  interpose  a  piece  of  coloured  glass  to  colour  the 
beam,  we  shall  resolve  that  coloured  light  into  two  con- 
stituents which  we  may  describe  as  "supplementary"  to 
one  another.  Blue  glass,  as  we  found  last  lecture, 
lets  some  green  and  violet  pass  as  well  as  blue ;  and 
here  again  you  see  the  fact  revealed.  Red  glass  on  the 
contrary  is  fairly  monochromatic;  for  though  we  split 
its  light  into  two  supplementary  beams,  both  are  red, 
scarcely  differing  in  hue  from  one  another. 

Natural  crystal  patterns,  produced  on  glass  by  pour- 
ing upon  it  some  crystallisable  solution  which  is  then 
allowed  to  dry,  form  objects  of  great  beauty.  Here,  for 
example,  is  a  plate  prepared  from  a  solution  of  anti- 
pyrin.  It  produces  an  effect  like  frost  on  the  window- 
pane.  But  the  delicate  traceries  and  plumes,  when 
placed  in  the  polariscope,  show  the  most  gorgeous  play 
of  colours,  as  you  see.  And  here  are  some  crystals  of 
sulphate  of  copper,  and  some  of  pyrogallic  acid,  which 
are  equally  curious. 

Now  there  are  in  nature  sundry  substances  besides 
crystals  which  possess  different  rigidities  in  different 
directions.  Thin  slices  of  wood,  for  example,  and  bone, 
and  horn,  and  many  other  animal  and  vegetable  structures. 
Here  is  a  thin  slice  of  horn.  It  is  nearly  transparent 
and  colourless.  But  if  put  into  the  polariscope,  with  its 
grain  inclined  at  45°  to  the  vertical,  you  see  at  once  the 
remarkable  streak  of  colour  which  it  produces.  Here 
again  is  a  quill-pen  flattened  out  and  mounted  as  a 
polariscope  object.  It  is  really  quite  gorgeous  in  its 
hues. 


in  POLARISATION  OF  LIGHT  151 

Here  is  a  very  interesting  object,  the  natural  lens 
from  the  eye  of  a  codfish.  Having  a  fibrous  and  radial 
structure  it  shows  a  black  cross  in  the  dark  field. 

Glass,  under  ordinary  circumstances,  is  devoid  of  any 
difference  in  rigidity  between  one  direction  and  another. 
Nevertheless,  if  it  is  suddenly  heated  or  suddenly  cooled, 
the  unequal  expansion  of  its  parts  produces  differences  in 
rigidity  which  make  themselves  visible  in  the  polariscope. 
Here,  for  example,  is  a  small  square  piece  of  glass,  which 
at  present  shows  no  colour  or  any  other  effect  when 
placed  in  the  polariscope.  But  if  it  is  dropped  into  a 
heated  brass  frame  which  will  quickly  warm  its  edges 
before  the  central  part  of  it  has  time  to  expand,  its 
structure  will  be  put  under  unequal  stresses,  and  the 
resulting  strains  will  show  themselves  in  the  strange 
patterns  of  colours  which  you  now  see  growing  into  sight 
upon  the  screen. 

If  a  hot  piece  of  glass  is  suddenly  chilled,  so  that  the 
outer  part  cools  and  contracts  before  the  inner  part  has 
time  to  cool,  the  piece  may  acquire  and  remain  in  a 
state  of  permanent  strain.  Such  glass,  usually  described 
as  "unannealed,"  is  very  liable  to  break1  with  almost 
explosive  violence.  Here  is  a  square  piece  of  glass,  of 
no  colour  in  itself,  but  which  has  been  suddenly  chilled. 
Its  state  of  permanent  strain  is  at  once  revealed  by  the 
peculiar  pattern  and  the  dull  tints  that  seem  to  form 
around  a  distorted  cross  radiating  from  its  centre.  A 

1  The  extreme  case  is  presented  by  "Rupert's  drops,"  which 
are  drops  of  melted  glass  suddenly  cooled  by  dropping  them  into 
water.  When  examined  in  polarised  light  (best  when  immersed  in 
a  small  glass  tank  filled  with  oil  to  obviate  surface  reflexions)  they 
show  fine  colours. 


152  LIGHT  LECT. 

second  square  of  glass  which  has  been  still  more  suddenly 
cooled  shows  the  same  black  cross  (Fig.  98),  but  the  tints 
in  the  corners  of  the  square  run 
up  into  the  second  order.  Here 
again  is  a  short  cylinder  of  glass 
which  was  suddenly  chilled  all 
round  its  outside.  The  peripheral 
surface  has  contracted  upon  the 
inner  part  and  compressed  it  with 
an  enormous  force.  As  a  result 
you  see  not  only  the  black  cross 

indicative  of  a  radial  disposition  of  the  axes  of  elasticity, 
but  a  number  of  concentric  rings  coloured  with  the  now 
familiar  succession  of  Newton's  tints  right  up  to  the 
fourth  order. 

And  now  I  am  going  to  squeeze  a  piece  of  glass 
mechanically,  by  gripping  it  in  a  strong  brass  frame  and 
then  forcing  a  point  against  its  side  by  turning  a  strong 
screw.  In  the  dark  field  the  glass  itself  shows  neither 
light  nor  colour,  until  I  put  on  the  screw.  But  so  soon 
as  compression  is  applied  a  luminous  pattern  at  once 
seems  to  grow,  stretching  off  in  two  patches  at  about  45° 
on  each  side  of  the  point  where  the  screw  point  has  been 
forced  against  the  glass.  Tightening  the  screw  makes 
the  internal  strain  greater,  and  the  pattern  more  brilliant. 
Loosening  the  screw  releases  the  strain,  and  the  glass 
resumes  its  ordinary  colourless  state.  So,  you  see,  you 
can  use  polarised  light  not  only  to  detect  false  gems  from 
real,  not  only  to  tell  glass  from  crystal,  but  also  to 
ascertain  whether  any  piece  of  glass  is  likely  to  break  or 
not.  Any  piece  of  glass  that  has  been  too  suddenly 


in 


POLARISATION  OF  LIG 


cooled,  that  is,  has  not  been  properly  annealed  by  slow 
cooling  down  from  the  furnace  heat,  can  always  be 
detected  by  the  colours  it  shows  when  placed  in  the 
polariscope  between  polariser  and  analyser. 

For  such  purposes  a  very  simple  polariscope,  such  as 
any  ingenious  boy  might  construct  for  himself,  is  quite 
sufficient.  Here  (Fig.  99)  is  such  a  polariscope,  made 
entirely  of  glass.  The  polariser  is  simply  a  flat  piece  of 
window-glass,  9  inches  long  by  5  inches  wide,  blackened 


with  black  varnish  on  its  under  side,  and  laid  down  on  a 
simple  frame  of  wood.  Two  other  pieces  of  glass  are 
cut  of  the  size  8J  by  5  inches.  One  of  these  is  of  clear 
window-glass,  the  other  of  ground  glass.  Across  the 
lower  part  of  the  clear  piece,  and  at  if  inch  from  its 
edge,  is  cemented  a  strip  of  glass  5  inches  long  by 
i  inch  broad,  to  serve  as  a  ledge  on  which  to  support 
the  objects  when  being  looked  at.  These  two  pieces  of 
glass  are  joined  at  the  top  by  a  hinge  of  paper  or  cloth 
cemented  to  them ;  and  they  stand  up,  like  a  roof,  over 


154  LIGHT  LECT. 

the  piece  of  blackened  glass,  being  kept  in  their  places 
on  the  baseboard  by  two  strips  of  wood  which  are 
fastened  to  the  board  9!  inches  from  one  another. 
The  baseboard  should  be  17  inches  long  by  5  inches 
wide.  Daylight  or  lamp-light  is  allowed  to  strike  upon 
the  ground  glass,  and  thence  passes  down  to  the 
blackened  glass,  is  reflected  at  an  incidence  of  about 
57°  to  its  surface,  and  so  passes  as  a  polarised  beam 
through  the  piece  of  clear  glass  on  its  way  to  the  eye. 
As  analyser,  seeing  that  Nicol  prisms  are  expensive,  a 
cheap  substitute  must  be  found.  One  that  is  quite  good 
enough  for  many  purposes,  may  be  made  by  taking  a 
bundle  made  of  eight  or  ten  very  thin  slips  of  glass,1 
each  about  ij  inch  long  and  f  inch  wide,  and  fixing 
them  with  sealing  wax  obliquely  across  a  small  wooden 
tube  or  box  with  open  ends.  They  should  be  fixed  in 
the  wooden  tube  so  that  the  glass  slips  are  inclined  at 
about  33°  to  the  direction  in  which  the  light  is  to  pass 
through  the  tube. 

With  quite  simple  apparatus  you  can  verify  and  repeat 
many  of  the  experiments  that  have  now  been  shown 
before  you.  There  are  many  others,  equally  beautiful, 
that  I  have  not  shown ;  for  in  a  single  lecture  one  can 
only  deal  very  incompletely  with  this  fascinating  and  com- 
plicated subject  of  polarisation.  I  have  not  shown  you 
how  quartz  crystals  possess  a  special  property  of  rotating 
the  polarised  light,  nor  have  I  told  you  how  solutions  of 
sugar  and  sundry  other  liquids  are  found  also  to  produce 

1  The  very  thin  glass  used  for  "covers"  for  microscopic  objects 
is  suitable.  It  is  usually  supplied  only  in  round  cover-disks.  But 
any  good  optician  could  procure  rectangular  slips  of  the  size  named. 


in  POLARISATION  OF  LIGHT  155 

an  optical  rotation.  Indeed,  the  regular  way  adopted 
in  sugar  factories  to  measure  the  amount  of  sugar  in  a 
watery  syrup  is  to  put  some  of  it  into  a  polariscope  and 
measure  how  much  it  turns  the  direction  of  the  vibra- 
tions. Lastly,  I  have  said  nothing  about  the  remarkable 
discovery  with  which  Faraday  crowned  his  researches  in 
this  place,  namely,  that  the  polarised  waves  of  light  can  be 
rotated  by  a  magnet.  Let  me  hope  that  some  day  you 
may  learn  of  these  marvellous  discoveries,  to  which  the 
things  you  have  seen  to-day  constitute  a  first  step. 


APPENDIX    TO    LECTURE    III 

THE    ELASTIC-SOLID    THEORY    OF    LIGHT 

ON  p.  34  it  is  remarked  that  light- waves  travel  slower  in 
denser  media;  and  on  p.  129  it  is  explained  how  in  a 
double-refracting  crystal  the  waves  are  split  into  two  sets 
which  travel  with  different  velocities.  It  is  expedient  to 
enter  further  into  the  question  of  the  velocity  of  propagation 
of  light-waves.  If  it  is  assumed  as  a  fundamental  point 
that  the  velocity  of  propagation  of  a  wave  is  equal  to  the 
square-root  of  the  elasticity  of  the  medium  divided  by  its 
density  (or,  as  expressed  in  symbols,  -v  —  v/  E-f-D,  which 
is  Newton's  law),  then  it  is  only  possible  to  account  for  the 
co-existence  of  two  different  velocities  by  supposing  that 
displacements  in  different  directions  either  evoke  a  different 
elasticity  or  call  into  operation  a  different  density.  But, 
since  the  medium  of  which  the  waves  constitute  light  is  the 
ether,  one  has  to  deal,  in  the  case  of  the  transmission  of 
light  through  crystals,  with  the  ether  as  it  exists  in  the 
crystal.  If  we  assume  that  the  ether  acts  as  an  in- 
compressible homogeneous  elastic  solid,  then  the  ordinary 
theory  of  elasticity  suffices  as  a  theory  of  the  ether.  For 
long  this  "  elastic-solid  theory  "  of  the  ether  has  held  sway, 
and  has  received  elaborate  mathematical  treatment  at  the 
hands  of  Green,  Fresnel,  MacCullagh,  Neumann,  Cauchy, 
and  others.  On  this  view  the  ether  particles  within  crystals 
are  arranged  differently  in  different  directions,  symmetrically 
with  respect  to  three  rectangular  axes,  and  therefore  the 
properties  of  the  ether  as  a  medium  for  transmitting  waves 
will  be  modified  by  the  presence  of  the  crystalline  matter. 


APP.  ELASTIC-SOLID  THEORY  OF  LIGHT  157 

But  here  a  difference  of  view  may  arise  ;  for  it  may  be  held 
(with  Fresnel)  that  the  presence  of  the  crystalline  matter 
modifies  the  density  of  the  ether  without  altering  its 
elasticity  ;  or  it  may  be  supposed  (with  MacCullagh  and 
Neumann)  that  the  presence  of  the  crystalline  matter 
modifies  the  elasticity  in  different  directions  without 
affecting  its  density.  In  either  case  the  assumptions  lead 
to  equations  that  fit  the  fundamental  facts  of  double-refrac- 
tion and  polarisation.  But  there  arises  this  difference  that 
whereas  the  theory  of  Fresnel  supposes  the  displacements 
to  occur  at  right  angles  to  the  so-called  "  plane  of  polarisa- 
tion,'5 that  of  MacCullagh  treats  them  as  executed  in  that 
plane.  As  to  the  actual  direction  in  which- the  displace- 
ments are  executed,  the  properties  of  tourmaline  suffice 
(apart  from  other  proofs)  to  determine  the  fact  that  in  the 
extraordinary  wave,  which  is  transmitted,  the  displacements 
are  executed  parallel  to  the  principal  axis  of  the  crystal, 
while  in  the  ordinary  wave,  which  is  absorbed,  the  displace- 
ments are  at  right  angles  to  that  axis.  The  simple  proof 
being  (see  Philosophical  Magazine,  August  1881)  that 
tourmaline  is  opaque  (at  least  in  thick  slices)  to  all  light 
travelling  along  the  principal  axis  of  crystallisation  ;  hence 
it  absorbs  those  vibrations  which  are  transverse  to  that 
axis.  (Compare  p.  119  above.) 

But  the  elastic-solid  theory  is  not  the  only  possible 
theory  of  light.  Instead  of  supposing  the  ether  to  be  itself 
modified  in  arrangement  or  properties  by  the  presence  of 
crystalline  matter  we  might  suppose  it  to  be  itself  isotropic, 
having  equal  elasticity  and  density  in  every  direction,  but 
that  in  its  motions  it  communicates  some  of  its  energy  to 
the  particles  of  matter  through  which  the  wave  is  travelling. 
If  the  particles  of  gross  matter  thus  load  the  ether  their 
vibration  will  during  the  passage  of  the  wave  take  up  some 
of  the  energy  and  retard  the  rate  at  which  the  group  of 
waves  can  travel.  We  should  then  have  a  difference 
between  the  velocity  of  propagation  of  the  individual  waves 
themselves  and  the  velocity  of  propagation  of  the  group  of 
waves  ;  and  in  that  case  the  velocity  of  propagation  of  the 
group — the  apparent  velocity  of  light — would  be  slower 


158  LIGHT  LECT.  in 

than  the  velocity  of  the  waves  themselves,  and  would  be 
different  for  waves  of  different  frequency.  This  is  in  fact 
the  phenomenon  of  dispersion.  In  the  case  of  crystalline 
media  the  retardation  and  the  dispersion  would  be  different 
in  different  directions,  and  would  depend  upon  the  direction 
of  the  displacements  with  respect  to  the  axes  of  the  crystal. 
But  as  to  the  connection  between  the  molecules  of  matter 
and  the  ethereal  medium  involved  in  such  theories,  very 
little  is  known,  and  there  is  room  for  many  different 
hypotheses  as  to  the  nature  of  such  connection.  Helmholtz, 
Kelvin,  Lommel,  Sellmeier  and  others  have  made  various 
suggestions  of  which  an  admirable  account  is  to  be  found 
in  Glazebrook's  "  Report  on  Optical  Theories,"  British 
Association  Report ',  1885. 

The  electromagnetic  theory  of  light  which  Maxwell 
founded  upon  the  basis  of  the  experimental  work  of  Faraday 
has  now  definitely  superseded  all  the  purely  mechanical 
theories.  Some  account  of  this  theory  is  given  in  the 
Appendix  to  Lecture  V  'p.  230). 

The  only  other  poi.  \l£it  need  claim  attention  here  is  the 
use  of  the  term  "  plane  of  polarisation."  This  term,  which 
is  variously  defined  by  different  writers,  is  used  to  denote  a 
plane  with  respect  to  which  the  polar  properties  of  the 
wave  can  be  described.  It  must  r~^essarily  contain  the 
line  along  which  th  wave  is  being  propagated  (i.e.  the 
"ray"  lies  in  this  p,  •**.) ;  but,  so  far  as  the  orientation  of 
this  plane  around  the  ray  is  concerned,  its  definition  with 
respect  to  the  polar  properties  is  purely  a  matter  of  conven- 
tion. The  following  is  Herschel's  definition  (Encyclopedia 
Metropolitana,  article  "Light,"  p.  506) — "The  plane  of 
polarisation  of  a  polarised  ray  is  the  plane  in  which  it  must 
have  undergone  reflexion,  to  have  acquired  its  character  of 
polarisation  ;  or  that  plane  passing  through  the  course  of 
the  ray  perpendicular  to  which  it  cannot  be  reflected  at  the 
polarising  angle  from  a  transparent  medium  ;  or,  again, 
that  plane  in  which,  if  the  axis  of  a  tourmaline  plate  exposed 
perpendicularly  to  the  ray  be  situated,  no  portion  of  the  ray 
will  be  transmitted."  If  we  refer  to  the  Nicol  prism  (Fig. 
84,  p.  123)  we  shall  see  that,  according  to  the  convention 


APP.  ELASTIC-SOLID  THEORY  OF  LIGHT  159 

thus  laid  down  by  definition,  the  plane  of  polarisation  of  the 
light  that  emerges  is  parallel  to  the  longer  diagonal  of  the 
end-face  •;  and  the  vibrations  are  executed  at  right  angles  to 
this.  To  avoid  periphrasis  in  these  Lectures  the  author 
speaks  of  the  plane  in  which  the  vibrations  are  executed  as 
the  plane  in  which  the  wave  is  polarised  (see  descriptions 
of  Figs.  69-73,  pp.  113-117). 


j  - 


LECTURE   IV 

THE    INVISIBLE    SPECTRUM    (ULTRA-VIOLET    PART) 

The  spectrum  stretches  invisibly  in  both  directions  beyond  the  visible 
part — Below  the  red  end  are  the  invisible  longer  waves  that 
will  warm  bodies  instead  of  illuminating  them — These  are 
called  the  calorific  or  infra-red  waves.  Beyond  the  violet  end 
of  the  visible  spectrum  are  the  invisible  shorter  waves  that 
produce  chemical  effects — These  are  called  actinic  or  idtra-violet 
waves — How  to  sift  out  the  invisible  ultra-violet  light  from  the 
visible  light — How  to  make  the  invisible  ultra-violet  light 
visible — Use  of  fluorescent  screens — Reflexion,  refraction,  and 
polarisation  of  the  invisible  ultra-violet  light — Luminescence  : 
the  temporary  kind  called  Fluorescence,  and  the  persistent 
kind  called  Phosphorescence — How  to  make  "  luminous  paint" 
• — Experiments  with  phosphorescent  bodies — Other  properties 
of  invisible  ultra-violet  light — Its  power  to  diselectrify  electri- 
fied bodies — Photographic  action  of  visible  and  of  invisible  light 
— The  photography  of  colours — Lippmann's  discovery  of  true 
colour-photography — The  reproduction  of  the  colours  of  natural 
objects  by  trichroic  photography — Ives's  photochromoscope. 

ALL  kinds  of  light  in  the  visible  spectrum  are  comprised 
between  the  extreme  red  at  one  end  and  the  extreme 
violet  at  the  other.  Their  wave-lengths  vary  between 
about  32  millionths  of  an  inch  (extreme  red)  and  15 
millionths  of  an  inch  (extreme  violet).  But  besides  the 
waves  of  various  colours,  between  those  limits,  which 


LECT.  iv  THE  INVISIBLE  SPECTRUM  161 

are  visible,  there  are  other  waves  that  bring  no  sensa- 
tion to  our  eyes,  which  are  invisible,  and  yet  are  light- 
waves. In  brief,  the  spectrum  extends  in  both  directions 
invisibly,  both  below  the  extreme  red  and  beyond  the 
extreme  violet. 

Perhaps  you  raise  the  objection  that  if  such  waves 
are  invisible  they  cannot  be  waves  of  light.  Well,  if 
you  were  to  lay  down  as  a  definition  beforehand  that 
the  term  "  light  "  must  be  applied  only  to  the  waves 
that  are  visible  to  the  human  eye,  there  is  nothing  more 
to  be  said.  But  what  if  there  are  other  eyes,  or  other 
processes  that  will  enable  these  waves  to  be  observed  ? 
Further,  if  it  is  found  that  these  invisible  waves  agree 
with  the  visible  waves  in  other  important  respects,  if,  in 
fact,  it  is  found  that  they  can  be  reflected,  refracted, 
polarised,  and  diffracted,  then  we  are  bound  to  regard 
them  as  light.  They  may  have  wave-lengths  that  are 
larger  than  that  of  the  red  waves,  or  smaller  than  that 
of  the  violet  waves,  and  so  our  eyes,  with  their  limited 
range  of  perception,  may  fail  to  be  sensitive  to  them. 
Nevertheless  if  in  their  physical  properties  they  agree 
with  the  visible  kinds,  then  the  fact  that  to  us  they 
are  invisible  simply  demonstrates  the  imperfection  of 
our  eyes.  Had  we  lived  all  our  lives  behind  screens  of 
red  glass  we  should  never  have  known  anything  of  green 
or  blue  waves :  we  should  have  been  blind  to  waves  of 
these  particular  kinds.  But  though  we  should  never 
have  seen  them  that  would  not  prove  that  they  were 
not  waves  of  light. 

Now  that  part  of  the  invisible  spectrum  which  con- 
sists of  waves  of  too  large  a  size — of  too  great  a  wave- 
Mi 


1 62  LIGHT  LECT. 

length — to  affect  our  eyes  possesses  another  property, 
namely,  that  of  warming  the  things  upon  which  it  falls. 
Some  of  the  visible  waves,  particularly  those  toward  the 
red  end  of  the  spectrum,  share  the  same  property,  but 
to  a  less  extent.  The  longer  invisible  waves  are  called 
variously  the  calorific  or  infra-red  waves.  We  shall  deal 
with  these  in  the  next  lecture.  At  the  other  end  of  the 
spectrum,  beyond  the  violet,  we  have  again  waves  which 
are  invisible  by  reason  of  being  of  too  small  a  size  to 
affect  our  sense  of  sight;  and  these  possess  several 
remarkable  properties.  They  are  active  in  producing 
certain  chemical  effects,  notably  those  known  as  photo- 
chemical or  photographic.  They  produce  certain 
physiological  effects  also  on  animal  and  vegetable 
tissues.  They  actively  provoke  in  certain  bodies  the 
property  of  shining  in  the  dark,  or  phosphorescence. 
Lastly,  they  have  certain  electrical  properties.  These 
short  waves  are  known  by  the  various  names  of  actinic, 
photographic,  or  ultra-violet  waves.  The  last  of  these 
terms  is  much  to  be  preferred.  Some  of  these  chemical 
effects  are  also  produced  by  visible  light,  especially  by 
the  blue  and  violet  waves.  Fig.  100  is  a  diagram  which 
gives  a  general  idea *  of  the  distribution  of  these  effects 
for  waves  of  different  lengths.  The  greatest  luminosity 
to  the  eye  is  possessed  by  waves  having  a  wave-length 
of  about  22  millionths  of  an  inch  or  0*00055  °f  a 
millimetre.  The  greatest  heating  effect  occurs  with* 
waves  of  about  40  millionths  of  an  inch,  or  0*001  of 

1  A  table  of  wave-lengths  and  frequencies  of  all  kinds  of  light 
from  the  lowest  infra-red  up  to  the  highest  ultra-violet  has  been 
added  as  an  Appendix  to  this  Lecture. 


iv  THE  INVISIBLE  SPECTRUM  163 

a  millimetre.  The  greatest  chemical 1  effect  occurs 
with  waves  of  about  i6|  millionths  of  an  inch,  or  about 
0*00041  of  a  millimetre. 

Now,  it  is  desirable  for  purposes  of  experiment  to 
separate  the  waves  which  can  produce  one  of  these 
effects  from  those  which  produce  another.  If  we  desire 
to  sift  out  the  ultra-violet  waves  from  all  other  kinds, 
there  are  several  courses  open  to  us.  Firstly,  we  may 


FIG.  ioo 

use  prisms  which  will  disperse  the  waves  and  sort  them 
out  into  a  spectrum  according  to  their  sizes.  Secondly, 
we  may  accomplish  the  same  thing  by  using  a  diffraction 
grating  to  produce  a  spectrum.  Or,  thirdly,  we  may 
employ,  as  a  means  of  sifting,  sheets  of  different  sub- 
stances that  have  the  power  of  absorbing  waves  of  one 
sort  while  transmitting  those  of  another.  This  last 
process  we  found  excellent  when  applied  to  visible  light, 

1  This  is  on  the  assumption  that  the  effect  is  measured  by  a 
particular  chemical  reaction,  viz.  the  darkening  of  chloride  of  silver. 
If  a  different  reaction,  say,  for  example,  the  darkening  of  ferro- 
prussiate  salts  ("  blue-prints  ")  were  taken  as  a  basis  of  measurement, 
the  maximum  effect  would  be  found  to  occur  at  some  other  point  in 
the  spectrum. 


164  LIGHT  LECT. 

for  by  using  a  red-coloured  glass  we  were  able  to  cut  off 
all  the  other  colours  and  leave  only  the  red.  Unfortu- 
nately no  perfect  filter-screen  exists  that  will  cut  off  all 
the  visible  light  and  yet  transmit  the  ultra-violet  waves. 
Glass  tinted  a  deep  violet  colour  with  manganese,  or  with 
manganese  and  cobalt,  may  serve  to  cut  off  most  of  the 
visible  light  while  transmitting  a  fair  proportion  of  ultra- 
violet waves,  mixed  with  some  violet  light.  For  many 
purposes  this  is  good  enough.  But,  unfortunately,  every 
kind  of  glass  cuts  off  the  extreme  part  of  the  ultra-violet 
light.  Even  the  lightest  crown  glass,  though  moderately 
transparent  to  waves  from  15  millionths  to  n  millionths 
of  an  inch  long  is  totally  opaque  to  all  waves  smaller  than 
1 1  millionths ;  while  dense  flint  glass  (containing  lead) 
is  opaque  to  everything  beyond  the  wave-length  of  13 '3 
millionths  of  an  inch.  Hence,  for  experiments  on  ultra- 
violet light  it  is  expedient  not  to  use  glass  lenses  or 
prisms,  provided  some  more  transparent  medium  can  be 
found.  Happily  both  quartz  and  fluor-spar  are  much 
more  transparent  to  ultra-violet  waves  than  glass  is. 
Quartz  transmits  them  down  to  about  8'i  millionths 
of  an  inch,  and  fluor-spar  down  to  8  millionths.  My 
lantern  is  on  this  occasion  provided  with  condensing 
lenses  of  quartz.  When  we  want  a  spectrum  we  will 
use  a  quartz  prism  and  a  focusing-lens  also  cut  from 
quartz  crystal. 

Let  me  now  proceed  to  demonstrate  some  of  the 
photographic  properties  of  light-waves.  Here  is  a  piece 
of  ordinary  "  printing-out "  paper,  that  is  paper  which 
has  been  covered  with  a  sensitive  film  impregnated  with 
chloride  or  bromide  of  silver,  which,  when  exposed  for 


iv  THE  INVISIBLE  SPECTRUM  165 

a  sufficient  time  to  light,  turns  nearly  black.  Over  this 
sheet  of  sensitised  paper  I  place  some  stencil-plates  cut 
out  in  sheet-zinc  ;  and  then  expose  it  to  the  white  light 
that  comes  from  an  electric  arc-lamp  on  the  table.  In 
half  a  minute  the  paper  will  have  darkened  sufficiently 
for  you  to  see  that  where  the  light-waves  have  fallen 
upon  the  exposed  parts  they  have  produced  the  chemical 
action,  and  have  printed  the  patterns  of  the  stencils. 
In  this  experiment  all  kinds  of  rays — calorific,  visible, 
and  actinic — have  been  allowed  to  fall  on  the  paper; 
but  which  of  them  were  the  agents  in  producing  the 
effect  ?  That  is  easily  tested.  We  turn  on  the  light  in 
the  optical  lantern,  using  the  quartz  lenses  and  prism  to 
produce  a  spectrum  for  us.  Then  along  the  whole 
length  of  the  visible  spectrum,  and  to  a  distance  into 
the  invisible  spectrum  at  both  ends,  we  stretch  out  a 
long  strip  of  sensitised  photographic  paper.  It  must  be 
left  there  for  several  minutes,  during  which  time  we 
may  investigate  another  point. 

Here  is  another  sheet  of  sensitised  paper.  Over  it 
I  lay  a  sheet  of  opaque  tin-foil,  through  which  there  have 
been  cut  a  number  of  holes.  Over  these  holes  are 
laid  a  number  of  thin  slices  of  various  materials:  (i) 
window  glass ;  (2)  flint  glass ;  (3)  red  glass ;  (4)  green 
glass  ;  (5)  blue  glass  ;  (6)  quartz ;  (7)  fluor-spar ;  (8)  rock- 
salt  ;  (9)  ebonite.  I  now  slide  the  whole  arrangement 
under  the  beams  of  the  arc-lamp,  which  is  set  to  throw 
its  whole  light  downwards.  If  any  of  these  materials 
cuts  off  the  active  waves  we  shall  find  it  out  at  once, 
for  the  paper  will  be  darkened  only  under  those  sub- 
stances that  are  transparent  to  the  photographic  rays. 


166  LIGHT  LECT. 

Two  minutes'  exposure  suffices  for  our  simple  test.  On 
bringing  out  the  sheet  you  will  note  that  ebonite  (which 
is  black)  and  red  glass  have  alike  stopped  off  the  whole 
of  the  photographic  rays.  Green  glass  has  stopped  off 
the  greater  part  of  them,  and  the  flint  glass  has  evi- 
dently not  transmitted  them  all.  But  under  the  blue 
glass,  the  quartz,  the  rock-salt,  the  fluor-spar,  and  the 
window  glass  the  paper  seems  to  have  darkened  about 
equally.  With  a  more  refined  test  we  should  discover 
differences  between  these  also.  One  fact  we  have 
proved,  which  is  of  practical  importance,  namely,  that 
red  light  does  not  affect  a  photographic  film  though  it 
affects  our  eyes.  Every  photographer  knows  this ;  for 
he  takes  advantage  of  it  in  using  ruby  glass  or  red- 
coloured  tissue  to  cover  the  windows  of  his  "dark-room," 
or  to  screen  the  lamp  by  whose  light  he  works  in  pre- 
paring and  developing  his  plates. 

Meantime  our  long  strip  of  sensitised  paper  has  been 
exposed  to  the  spectrum,  and  now,  examining  it,  we  see 
that  it  has  sensibly  darkened  at  the  violet  end  and 
beyond  the  end  of  the  visible  violet  to  some  distance 
into  the  region  where  our  eyes  see  nothing;  in  short, 
the  photographic  spectrum  lies  mostly  beyond  the  violet, 
the  most  active  waves  being  shorter  than  any  that  are 
visible.  But  we  must  not  forget  that  with  other  chemical 
preparations  the  range  of  sensitiveness  can  be  changed. 
To  Captain  Abney  science  owes  the  introduction  of 
emulsions  of  bromide  of  silver  in  films  of  gelatine,  pre- 
pared in  such  a  way  as  to  be  sensitive  not  only  to  violet 
light  or  ultra-violet,  but  also  to  green,  to  yellow,  and 
even  to  red  waves. 


IV 


THE  INVISIBLE  SPECTRUM 


167 


Another  chemical  effect  which  light-waves  can  pro- 
duce is  to  cause  mixed  hydrogen  and  chlorine  gases  to 
enter  into  combination.  These  gases  (prepared  by 
electrolysis  of  hydrochloric  acid)  may  be  kept  mixed, 
but  not  chemically  combined  with  one  another,  in  glass 
bulbs  for  any  length  of  time,  provided  they  are  kept  in 
the  dark.  If  exposed  to  the  diffused  light  of  a  room 
they  slowly  combine.  But  if  exposed  to  direct  sunlight 
or  to  the  light  of  the  arc-lamp  they  combine  with  extra- 
ordinary violence  and  explode  the  bulb.  Again,  the 
question  arises:  which  part  of  the  light  is  it  that  produces 
the  effect  ?  Certainly  not  the  red  waves,  for  these  bulbs 
of  mixed  gas  may  be  exposed  freely  if  protected  by  red 
glass  and  will  not  explode.  The  active  kind  of  waves  is 
in  this  case  also  the  ultra-violet  kind. 

A  thin  glass  bulb  containing  the  mixed  gases  is  now 
taken  by  my  assistant 
from  a  tin  box,  where 
it  has  been  kept  in  the 
dark.  To  prevent  acci- 
dents he  places  it  in  an 
empty  lantern  (Fig.  101), 
into  the  nozzle  of  which 
we  will  direct,  from  out- 
side, the  beams  of  an 
electric  arc -lamp.  To 
cut  off  the  bulk  of  the 
ordinary  light  I  inter- 
pose first  a  sheet  of  violet  glass,  which  allows  only  violet 
and  ultra-violet^  to  pass.  Then,  interposing  a  quartz 
lens,  I  concentrate  the  beam  upon  the  bulb,  when — bang 


1 68  LIGHT  LECT. 

- — it  explodes,  demonstrating  the  activity  of  waves  of  this 
sort. 

Perhaps  it  may  have  struck  some  of  you  that  if  so 
great  a  photographic  activity  is  possessed  by  waves  that 
are  invisible  to  our  eyes,  it  ought  to  be  within  the  limits 
of  possibility  to  photograph  things  that  are  invisible. 
And  so  it  is.  It  is  now  some  twenty  years  since  a 
lecture  was  delivered  in  this  theatre  on  the  photography 
of  the  invisible  by  the  veteran  chemist,  Dr.  J.  Hall 
Gladstone,  who  succeeded  in  photographing  images  of 
things  quite  invisible  to  the  eye.  Behind  me,  against 
the  wall,  stands  a  drawing-board  covered  with  a  white 
sheet  of  cartridge  paper.  The  light  of  the  arc-lamp 
shines  on  it.  You  see  merely  a  white  surface.  The 
photographer,  Mr.  Norris,  has  brought  his  camera  here 
and  he  will  now  take  a  photograph  of  it.  When  he 
develops  the  photograph  you  will  find  that  the  photo- 
graph will  reveal  the  fact  that  an  inscription  has  been 
written  upon  the  sheet,  which,  though  invisible  to  you, 
can  be  photographed  by  the  camera.1 

Since  photographic  action  serves  to  detect  these 
ultra-violet  waves,  even  in  the  absence  of  all  kinds  of 
visible  light,  it  may  be  used  in  the  further  exploration  of 
the  properties  of  these  invisible  waves.  We  might  apply 
photographic  plates  to  prove  the  possibility  of  the  re- 

1  The  inscription  was  written  on  the  sheet  with  colourless  sulphate 
of  quinine  dissolved  in  a  solution  of  citric  acid.  This  substance 
fluoresces,  and  in  the  act  of  fluorescing  destroys  the  ultra-violet  light, 
which  would  otherwise  be  reflected  from  the  parts  of  the  paper  so 
treated.  The  parts  where  the  sulphate  of  quinine  has  been  applied 
consequently  come  out  in  the  photograph  darker  than  the  untouched 
surface  of  the  paper. 


iv  THE  INVISIBLE  SPECTRUM  169 

flexion  and  refraction  of  these  waves,  as  well  as  of  their 
interference  and  polarisation.  There  exists,  however, 
another  and  more  ready  method  of  investigation,  to 
which  we  will  now  proceed. 

Instead  of  photographing  the  invisible  we  may  make 
it  visible  to  the  eye  by  applying  the  discoveries  of 
Herschel,  Brewster,  and  Stokes.  There  are  a  number 
of  solid  substances,  such  as  fluor-spar,  uranium  glass,  and 
also  of  liquids,  such  as  petroleum,  solutions  of  quinine, 
and  of  many  of  the  dye-stuffs  derived  from  coal-tar,  which 
present  the  appearance  of  a  surface -colour  different 
from  that  of  the  interior.  Thus  quinine  is  colourless, 
but  shows  a  fine  blue  tint  on  the  surface  exposed  to  the 
light.  Uranium  glass  is  itself  yellow,  but  has  a  splendid 
green  surface-tint.  The  fact  is  that  these  substances 
have  the  property  of  absorbing  the  very  short  waves  of 
ultra-violet  light  and  transforming  them  into  waves  of 
longer  length  that  are  visible  to  our  eyes.  To  this 
phenomenon  Stokes  gave  the  name  of  fluorescence.  Let 
us  see  a  few  of  the  principal  .cases. 

From  the  optical  lantern,  provided  for  the  present 
experiments  with  quartz  lenses,  a  beam  of  light  streams 
forth.  Over  the  nozzle  of  the  lamp  is  now  placed  a 
cap  of  dark  violet  glass  to  cut  off  all  the  visible  light 
except  a  little  violet  that  unavoidably  accompanies  the 
invisible  ultra-violet  waves.  This  beam  is  directed 
upon  a  cube  of  uranium  glass ;  which  transmutes  the 
invisible  waves  into  a  brilliant  green.  And  you  see  the 
glass  cube  standing  out  vividly  in  the  darkness.  I  hold 
in  the  beam  a  bottle  of  paraffin  oil — it  seems  brilliantly 
blue.  A  green  decoction  of  spinach  leaves  (boiled  first 


LIGHT 


LECT. 


in  water,  then  dried,  and  lastly  extracted  with  ether) 
exhibits  a  strange  blood-red  fluorescence  on  its  surface. 

Here  is  a  row  of  specimen  bottles  containing 
fluorescent  liquids.  Yellow  fluorescein  gives  a  splendid 
green  fluorescence ;  pale  pink  eosin  (made  by  diluting 
red  ink)  gives  an  orange  fluorescence.  A  crimson 
solution  of  magdala-red  gives  a  scarlet  fluorescence; 
and  colourless  quinine  gives  its  characteristic  surface- 
blue. 

These  things  may  be  even  more  strikingly  shown 
by  reflecting  the  ultra-violet  beam  down  into  a  tall  glass 

cylinder  filled  with 
fluorescent  liquid. 
A  quartz  lens  placed 
just  above  the  jar 
(Fig.  102)  serves  to 
concentrate  the  beam 
into  a  sharp  cone  of 
colour.  I  take  a 
second  jar  filled 
simply  with  dilute 
ammonia-water,  and 
project  the  beam 
down  it.  Then  I 
sprinkle  into  the 
water  a  few  grains 
of  dry  fluorescein.  As  they  dissolve  there  descend  to 
the  bottom  curling  wreaths  of  bright  green  hue  and 
indescribable  beauty.  A  few  chips  of  horse-chestnut 
bark,  or  of  ash  bark,  would  yield  similar  effects. 

And   now,  perhaps,   you  will  appreciate  the  secret 


Quart*  Le 


FIG.  102. 


iv  THE  INVISIBLE  SPECTRUM  171 

of  the  photography  of  the  invisible.  The  inscription 
painted  on  the  white  sheet  was  painted  with  a  solution 
of  quinine.  You  shall  see  for  yourselves  the  invisible 
inscription ;  for  I  have  only  to  cast  upon  it  a  beam  of 
ultra-violet  light  to  cause  the  parts  painted  with  quinine 
to  shine  out  in  pale  blue  amid  the  darkness. 

Here  are  some  other  sheets  of  card  on  which 
fluorescent  patterns  have  been  painted.  On  one  of 
these,  side  by  side,  are  two  fleurs-de-lys,  which  in  day- 
light appear  to  be  equally  yellow.  One  is  painted  in 
common  gamboge,  the  other  in  fluorescein.  But  when 
I  turn  upon  them  the  beam  of  the  lamp  filtered  through 
dark  blue  glass,  the  whole  card  looks  deep  violet,  one 
of  the  lilies  seeming  black,  the  other  luminous  and 
greenish.  Another  card,  when  viewed  in  ordinary  white 
light  seems  to  be  merely  yellcw  all  over :  but  as  part 
of  the  yellow  surface  is  painted  in  gamboge,  and  the 
other  fluorescein,  the  effect,  when  examined  in  the 
ultra-violet  beam  is  to  give  a  black  pattern  on  a  bright 
ground. 

Twenty  years  ago  when  the  late  Professor  Tyndall 
was  delivering  in  the  United  States  his  famous  series  of 
lectures  on  light,  he  received  from  President  Morton, 
of  the  Stevens  Institute  at  Hoboken,  some  samples 
of  a  new  fluorescent  hydrocarbon,  "thallene,"  prepared 
from  petroleum  residues.  Some  large  sheet  diagrams 
of  flowers,  painted  in  parts  with  thallene  and  other 
fluorescent  materials,  were  amongst  the  objects  which 
Professor  Tyndall  brought  back  to  London.  These 
have  been  carefully  preserved  in  the  Royal  Institution, 
and  I  am  able  to  show  you  them  in  all  their  beauty. 


172  LIGHT  LECT. 

One  of  them  represents  a  wild  mallow,  the  leaves  being 
coloured  with  some  substance  which  fluoresces  green, 
whilst  the  flowers  have  a  pale  purple  fluorescence. 
The  effect  of  throwing  on  this  object  light  that  has 
passed  through  a  dark  blue  or  dark  violet  glass  is  very 
striking. 

Of  all  substances,  however,  that  are  known  to  me, 
the  most  highly  fluorescent  is  a  rather  expensive 
crystalline  product  called  by  chemists  the  platino- 
cyanide  of  barium.  In  ordinary  light  it  looks  l?fee  a 
pale  yellow  or  greenish  yellow  powder,  closely  resembling 
powdered  brimstone.  When  a  piece  of  paper  covered 
with  this  substance  is  held  in  the  ultra-violet  beam  it 
emits  a  yellowish-green  light  far  surpassing  in  brilliance 
that  emitted  by  uranium  glass  or  by  fluorescein.  •  Here 
is  a  small  fluorescent  screen  of  platino  -  cyanide  of 
barium  that  has  been  in  my  possession  sorne^  sixteen 
years. 

Now,  having  so  excellent  a  means  of  making  ultra- 
violet waves  visible,  let  us  apply  the  fluorescent  screen,  as 
Stokes  did  in  1851,  to  explore  the  ultra-violet  spectrum. 
My  assistant  puts  up  the  quartz  prism  in  front  of  the 
slit  to  give  us  once  more  the  spectrum.  Taking  a  long 
sheet  of  cardboard  that  has  been  painted  over  with 
quinine,  I  hold  it  so  that  the  spectrum  falls  upon  the 
middle  of  the  prepared  surface.  And  now  you  see  that 
the  spectrum  stretches  visibly  away  beyond  the  violet 
end,  for  here,  crossed  by  several  transverse  patches  of 
brighter  light,  the  ultra-violet  spectrum  comes  into  view 
as  a  pale -blue  extension.  Substituting  a  sheet  of 
uranium  glass  we  note  a  similar  extension  visible  into 


THE  INVISIBLE  SPECTRUM 


the  ultra-violet  to  a  distance  that  makes 
this  part  of  the  spectrum  seem  quite 
twice  as  long  as  the  whole  visible  part. 
Here,  best  of  all,  is  a  fluorescent 
screen  covered  with  platino-cyanide  of 
barium.  And  now  we  see  the  "long 
spectrum,"  stretching  away  to  three 
or  four  times  the  length  of  the  visible 
part  from  red  to  violet.  If  placed 
at  trie  other  end,  below  the  red, 
these  fluorescent  screens  show  no- 
thing whatever.  They  are  excited 
into  luminous  activity  not  by  the 
long  waves,  but  by  the  very  short 
ones. 

Let  us  then  avail  ourselves  of  the 
luminous  quality  of  the  fluorescent 
screen  to  examine  afresh  the  differ- 
ent degrees  in  which  transparent 
substances  transmit  or  absorb  these 
ultra-violet  waves.  The  ultra-violet 
part  of  the  spectrum  now  falls  upon 
the  screen,  the  surface  of  which  is 
thereby  stimulated  into  emitting  its 
fine  greenish  light.  Across  the  path 
of  the  invisible  beam  I  interpose  a 
piece  of  window  glass.  The  light  is 
dimmed  but  not  extinguished.  A 
piece  of  flint  glass  cuts  it  off  alto- 
gether; a  piece  of  blue  glass  dims 
it,  but  does  not  cut  it  off;  while  a 


FIG.  103. 


174  LIGHT  LECT. 

piece  of  red  glass  proves  to  be  absolutely  opaque. 
A  slice  of  quartz  crystal  is  fully  transparent ;  one 
of  calc-spar  rather  less  so,  while  a  thin  film  of  yellow 
gelatine  is  quite  opaque.  '  These  experiments  con- 
firm those  we  made  by  the  use  of  photographic 
paper. 

And  now  in  a  very  few  moments  we  can  demonstrate 
reflexion  and  refraction  of  the  ultra-violet  waves.  I 
place  my  fluorescent  screen  in  a  position  where  none  of 
the  waves  fall  upon  it.  Then  holding  a  mirror  in  the 
invisible  beam  I  reflect  ultra-violet  waves  upon  the 
screen,  which  at  once  shines  with  its  characteristic 
greenish  tint.  To  prove  refraction  I  interpose  in  the 
invisible  beam  a  quartz  prism,  which  deviates  ultra-violet 
waves  upon  the  fluorescent  screen,  and  again  it  shines. 
Polarisation  may  be  proved  by  using  two  Nicol  prisms 
precise^  aa  'was  done  in  my  last  lecture  for  ordinary 
light. 

This  phenomenon  of  fluorescence  is  only  one  of  a 
number  of  kindred  phenomena,  now  generally  classified 
together  under  the  name  of  Luminescence.  This  name 
was  given  by  Profes^r  E.  Wiedemann  to  all  those  cases 
in  which  a  body  is  caused  to  give  out  light  without 
having  been  raised  to  the  high  temperature  that  would 
correspond  to  the  ordinary  emission  of  light.  To  make 
ordinary  solids  red-hot  they  must  be  raised  to  between 
400°  and  500°  of  the  centigrade  scale  of  temperature. 
To  make  them  white-hot — that  is  to  say,  to  cause  them 
to  ^mit  not  only  red,  orange,  and  yellow,  but  also  green, 
blue,  and  violet  light,  they  must  be  raised  to  800°  or 
1000°  of  temperature.  At  red-heat  a  body  emits  few  or 


IV 


THE  INVISIBLE  SPECTRUM 


175 


no  green,  blue,  or  violet  waves.  But  as  we  have  seen  in 
the  examples  of  fluorescence  some  substances  while  quite 
cold  can  be  stimulated  into  giving  out  light  by  letting 
invisible  ultra-violet  waves  fall  upon  them.  So  we  may 
well  inquire  what  other  cases  there  are  of  the  emission 
of  cold  light.  Accordingly,  on  the  table  before  you 
there  are  enumerated  the  various  cases  of  lumin- 
escence. 


Phenomenon. 


Substance  in  which  it  occurs. 


[.   Chemi-luminescence 


2.   Photo-luminescence  : 

(n)  transient—  Fluorescence 


(b}  persistent  =  Phosphores- 
cence 


3.  Thermo-luminescence    . 

4.  Tribo-luminescence 


5.   Electro-luminescence  : 

(a)  Effluvio-luminescence 


(b)  Kathodo-luminescence 

6.  Crystallo-luminescence  . 

7.  Lyo-luminescence 

8.  X-luminescence     , 


Phosphorus  oxidising  in  moist 
air  ;  decaying  wood  ;  decay- 
ing fish  ;  glow-worm  ;  fire- 
fly; marine  organisms,  etc. 

Fluor-spar  ;  uranium  -  glass  ; 
quinine  ;  scheelite  ;  platino- 
cyanides  of  "  -io"-  bases ; 
eosin  and  many  coai-tar  pro- 
ducts. 

Bologna-stone ;  Canton's  phos- 
phorus and  other  sulphides 
of  alkaline  earths ;  some 
diamonds,  etc. 

Fluor-spar  ;  scheelite. 

Diamo:  Js  ;  sugar  ;  quartz  ; 
ur^  yl  nitrate  ;  pentadecyl- 
paratolylketone. 

Many  rarefied  gases ;  many 
of  the  fluorescent  and 
phosphorescent  bodies. 

•Rubies  ;  glass  ;  diamonds  ; 
many  gems  and  minerals. 

Arsenious  acid. 

Sub  -  chlorides  of  alkali- 
metals. 

Platino-cyanides ;  scheelite,  etc. 


1 76  LIGHT  LECT. 

The  Chemi-luminescence  which  heads  the  list  in- 
cludes those  cases  in  which  the  emission  of  cold  light 
is  accompanied  by  chemical  changes.  The  best  known 
instance  is  the  shining  in  the  dark  of  phosphorus  when 
slowly  oxidising  in  moist  air.  Lucifer  matches,  if 
damped  and  then  gently  rubbed,  shine  in  the  dark. 
The  best  way  to  show  this  is  to  take  a  sheet  of  ground 
glass,  dip  it  into  warm  water,  and  then  write  upon  its 
roughened  surface  with  a  stick  of  phosphorus,  which, 
for  the  sake  of  safety,  is  held  in  a  wet  cloth.  See  how, 
on  lowering  the  lights  in  the  theatre,  the  inscription  I 
have  scribbled  upon  the  glass  shines  with  a  pale  blue 
glimmer.  In  a  few  minutes  the  film  of  phosphorus  will 
have  oxidised  itself  completely,  and  the  emission  of 
light  will  be  at  an  end.  Curiously  enough,  this  light 
itself  consists  not  only  of  the  blue  waves  that  you  can 
see,  but  of  some  invisible  waves  also,  which  have  photo- 
graphic properties,  and  can,  like  Rontgen's  rays,  affect 
a  photographic  plate  that  is  enclosed  behind  an  opaque 
screen  of  black  paper.  It  is  now  known  that  the 
emission  of  light  by  glow-worms  and  fire-flies,  and  by 
the  innumerable  species  of  marine  creatures  and  deep- 
sea  fishes  that  shine  in  the  dark,  belongs  to  the  class 
of  chemi-luminescence.  So  does  the  emission  of  light 
by  the  microbes  that  are  developed  in  decaying  fish  and 
in  rotten  wood.  In  all  these  cases  there  is  chemical 
decomposition  at  work. 

Under  the  next  heading — Photo-luminescence — are 
included  those  cases  in  which  bodies  give  out  cold  light 
under  the  stimulation  of  light -waves.  Of  this  phe- 
nomenon there  are  two  cases.  Fluorescence  is  one, 


iv  THE  INVISIBLE  SPECTRUM  177 

and  in  that  case  the  emission  of  light  is  temporary, 
lasting  only  while  the  stimulation  lasts.  The  other 
case  is  that  known  as  Phosphorescence,  a  term  applied 
to  those  instances  in  which  the  emission  of  light  persists 
after  the  stimulation  has  ceased.  The  earliest  known 
example  of  phosphorescence  is  that  of  the  celebrated 
Bologna  stone.  A  shoemaker  in  the  city  of  Bologna, 
Casciarolo  by  name,  about  the  year  1602  discovered 
a  way  of  preparing  a  species  of  stone  which,  after 
having  been  exposed  to  sunlight,  would  shine  in 
the  dark.  This  was  done  l  by  the  partial  calcination 
of  "  heavy-spar  " — the  sulphate  of  barium — found  near 
that  city.  Here  is  a  small  sample  on  the  table.  Since 
that  time  many  other  substances  have  been  found  to 
possess  the  same  property.  Some  diamonds,  as  Robert 
Boyle  observed,  have  this  property.  And  amongst 
artificial  substances  the  sulphide  of  calcium  (Canton's 
"phosphorus")  and  sulphide  of  strontium  possess  the 
property  to  a  very  high  degree.  Sulphide  of  calcium 
can  be  prepared  by  pounding  up  oyster -shells  and 
heating  them  to  redness,  mixed  with  a  little  brimstone, 
in  a  closed  crucible.  The  addition  of  small  quantities 
of  other  materials — a  little  bismuth,  or  manganese,  or 
copper — has  a  remarkable  influence  in  aiding  the 
production  and  in  changing  the  colour  of  the  light 
emitted.  The  substance  sold  as  Balmain's  luminous 
paint  is  a  preparation  mainly  of  sulphide  of  calcium 
with  a  trace  of  bismuth.  Of  all  these  artificial  phos- 

1  See  a  singular  little  volume  published  in  Rome  in  1680,  by 
Marc'  Antonio  Cellio,  with  the  title  //  Fosforo,  o1  vero  /a  pietra 
Bolognese,  preparata  per  rilucerefrh  Fombra. 

N 


178  LIGHT  LECT. 

phori  the  most  powerful  by  far  is  a  new  luminescent 
paint  prepared  by  Mr.  Home. 

Behind  me  an  electric  lamp  is  arranged  to  throw  a 
beam  of  light  down  a  tube.  At  the  bottom  of  this 
tube  I  expose  to  the  stimulation  of  its  beams  a  few  of 
these  phosphorescent  stuffs.  Here  is  the  bit  of  Bologna 
stone.  On  removing  it  from  the  beam  it  shines  in  the 
dark,  but  not  nearly  so  brightly  as  the  bit  of  Home's 
new  material,  the  light  of  which  is  equal  to  about  one- 
tenth  of  a  candle  for  each  square  inch  of  surface  exposed. 
One  can  see  to  read  print  by  the  light  of  a  bit  of  this 
stuff.  I  have  heard  of  people  using  a  glow-worm  in 
the  same  way  in  order  to  read  at  night.  Here  is  a 
diamond  ring l  having  five  fine  diamonds.  On  exposing 
it  for  a  minute  to  the  light,  and  then  bringing  it  out 
into  .ne  darkness,  two  of  the  diamonds  are  seen  to 
shine  like  little  glow-worms. 

Here  is  a  box  containing  a  row  of  glass  tubes,  in 
each  of  which  is  a  white  powder.  These  powders  are 
phosphorescent.  But  first  they  must  be  stimulated, 
for  at  present  they  emit  no  light.  Let  us  expose  them 
for  thirty  seconds  to  the  beams  of  the  arc -lamp.  On 
then  bringing  them  into  the  darkness  of  the  theatre  it 
will  be  seen  that  they  glow  brightly  in  all  the  colours 
of  the  rainbow. 

Here,  again,  is  a  large  sheet  of  glass  which  has  been 
painted  over  with  luminous  paint.  I  lay  my  hand 
against  it,  and  expose  it  for  a  minute  to  the  beams  of 
the  arc -lamp.  Extinguishing  the  light,  you  see  the 
whole  sheet  splendidly  luminous,  save  where  the  shadow 
-1  The  property  of  Dr.  J.  H.  Gladstone,  F.R.S. 


iv  THE  INVISIBLE  SPECTRUM  179 

of  my  hand  appears  as  a  black  silhouette.  In  this  case 
the  luminescence  is  at  first  of  a  fine  blue  tint.  In  a 
few  minutes  as  it  fades  out  it  becomes  whiter ;  but  it 
will  go  on  all  night  giving  out  a  faint  light,  and  even 
then  will  not  have  yielded  up  its  whole  store  of  luminous 
energy.  Even  after  having  been  kept  six  weeks  in 
darkness  a  sheet  of  luminous  paint  will  still  emit  waves 
that  will  fog  a  photographic  plate.  If  one  takes  a 
sheet  of  luminous  paint  that  has  been  exposed  to  light, 
and  of  which  the  phosphorescence  has  already  died 
away,  one  finds  that  merely  warming  it  will  cause  it  to 
shine  more  brightly,  though  afterwards  it  is  darker. 
Here  is  such  a  sheet.  I  place  my  hand  against  the 
back,  and  you  note  that  where  my  hand  has  warmed  it 
it  shines  more  brightly.  If  one  makes  the  converse 
experiment  of  chilling  a  sheet  of  luminous  pain1,,  while 
it  is  phosphorescing,  one  finds  its  light  dimmed1,  but  it 
grows  brighter  while  being  warmed.  Professor  Dewar 
has  made  the  curious  discovery,  that  when  cooled  to  a 
temperature  of  about  200°  below  zero  in  liquid  air, 
many  substances  become  phosphorescent  that  are  not 
so  at  ordinary  temperatures.  Thus  he  has  shown  in 
this  theatre  how  such  things  as  feathers,  ivory,  and 
paper  become  highly  phosphorescent  on  being  cooled 
to  these  low  temperatures  and  then  illuminated.  They 
seem  when  chilled  to  acquire  the  power  of  absorb- 
ing luminous  energy  and  storing  it  for  subsequent 
emission  when  warmed.  The  analogies  between  these 
properties  and  those  of  luminous  paint  are  most 
suggestive.  A  sheet  of  luminous  paint  which  has 
been  exposed  and  cooled  becomes  a  veritable  lamp 


i8o  LIGHT  LECT. 

of  Aladdin.  One  has  but  to  warm  it  by  the  hand  and 
it  shines. 

Here  we  touch  upon  the  third  sort  of  luminescence 
named  in  our  list  (p.  175),  namely  Thermo-luminescence. 
This  term  is  applied  to  the  property  possessed  by 
various  minerals,  particularly  by  the  green  sorts  of  fluor- 
spar, to  shine  in  thejdark  on  being  heated.  Over  a 
large  atmospheric  gas-burner  a*  square  of  sheet-iron  has 
been  heated  to  near  redness.  Upon  this  hot  surface, 
invisible  in  the  darkness,  I  scatter  out  of  a  pepper-box 
some  fine  fragments  of  crushed  fluor-spar.  As  they 
heat  up  they  shine  like  little  glow-worms.  They  shine 
brightly  for  a  few  minutes,  then  fade,  but  would  continue 
for  several  hours  to  emit  a  faint  glow.  After  having 
been  once  thus  heated  they  seem  to  have  lost  their 
store  of  luminous  energy,  for  on  a  second  heating  they 
do  not  again  luminesce.1 

The  term  Tribe-luminescence,  which  stands  next  on 
the  list,  relates  to  the  production  of  luminescence  by 
friction.  There  is  a  very  simple  experiment  that  can 
be  tried  at  home  without  any  apparatus.  Crush  a  lump 
of  sugar  in  a  perfectly  dark  room.  In  the  act  of  being 
crushed  it  emits  a  pale  luminescence.  So  do  crystals  of 
uranium  nitrate  if  shaken  up  in  a  bottle,  or  triturated  in 
a  mortar.  Let  me  show  it  to  you  on  a  larger  scale. 

1  Many  other  minerals  have  similar  powers.  In  some  cases  the 
power  of  thermo-luminescing  can  be  revivified  by  fresh  exposure  to 
light,  or  by  stimulation  by  an  electric  spark  or  by  kathode  rays. 
Wiedemann  has  found  artificial  substances  that  are  thermo- 
luminescent,  and  in  particular  a  preparation  of  sulphate  of  calcium 
having  intermingled  as  a  "solid  solution"  a  small  percentage  of 
sulphate  of  manganese. 


IV 


THE  INVISIBLE  SPECTRUM 


181 


Here  is  a  large  specimen  of  quartz  crystal  weighing 
nearly  a  hundred  pounds.  One  of  its  faces  is  almost 
flat.  Taking  a  smaller  crystal  of  quartz  in  my  hand  I 
rub  it  to  and  fro  upon  the  larger  crystal.  You  can  all 
see  the  brilliant  flashes  that  are  emitted  in  the  operation. 
"^Reserving  for  discussion  in  my  final  lecture  the  use 
of  electric  discharges  to  produce  luminescence,  we  will 
return  to  the  properties  of  the  ultra-violet  light.  One 
effect  which  they  possess  above  all  other  kinds  of  light 
is  that  of  producing  diselectrification  of  electrified 
bodies,  a  phenomenon  discovered  by  the  late  Professor 
Hertz.  But  there  is  this  peculiar  limitation.  If  the 
electrified  surface  is  that  of  a  metal  surrounded  by  air, 
then  when  ultra-violet  light  falls  upon  it  it  will  produce 
diselectrification  if  the  surface  is  negatively  electrified, 
but  not  if  the  electrification  is  of  positive  sign.1 

The  fundamental  point  is  easily  shown.  Here  is 
an  electroscope  made  with  two  leaves 
of  aluminium  mounted  on  either  side 
of  a  central  blade  of  aluminium,  and 
enclosed  (Fig.  104)  in  a  thin  glass 
jar.  To  the  top  of  the  stem  is  affixed 
a  disk  of  sheet  zinc  which  has  been 
freshly  cleaned  with  a  little  sodium- 
amalgam.  It  is  bent  back  at  45°, 
at  which  incidence  the  results  are 
most  favourable.  I  hold  near  the 
zinc  disk  a  rubbed  glass  rod,  and  touch  the  disk  while 

1  I  have,  however,  found  that  a  surface  of  peroxide  of  lead  sur- 
rounded by  an  atmosphere  of  hydrogen  is  diselectrified  if  the 
electrification  is  positive. 


FIG.  104. 


182  LIGHT 


LECT. 


it  is  under  the  influence  of  the  positive  charge  of  the 
glass.  The  electroscope  thus  acquires  by  influence  a 
negative  electrification,  and  the  aluminium  leaves  stand 
out  at  a  sharp  angle.  Now  throwing  a  beam  of  ultra- 
violet light  upon  the  disk,  the  leaves  are  seen  to  collapse 
rapidly.  If  the  electroscope  is  positively  electrified,  the 
leaves  do  not  fall  down  when  the  beam  of  ultra-violet 
light  is  directed  upon  the  disk.  Even  the  longer  waves 
of  visible  light  are  active  on  a  clean  surface  of  sodium 
or  potassium.  The  different  kinds  of  light-waves  have 
different  photo-electric  powers  as  well  as  different  photo- 
chemical powers. 

At  the  beginning  of  this  lecture  I  dwelt  upon  the 
photographic  actions  of  light-waves,  and  I  return  now 
to  this  topic  in  order  to  speak  of  the  problem  which 
has  of  late  aroused  such  keen  interest  amongst  scientific 
photographers,  namely,  the  photography  of  colours. 
Many  have  been  the  attempts  to  produce  true  photo- 
graphs of  things  in  their  natural  colours.  All  hope  of 
this  was  vain  so  long  as  photographers  worked  with 
chemically  prepared  plates  that  were  more  sensitive  to 
the  invisible  light  than  to  the  visible  kinds.  Further, 
in  the  old  collodion  processes  the  greater  sensitiveness 
of  the  chemicals  to  blue  and  violet  waves,  and  their 
relative  insensitiveness  to  orange  and  red  light,  caused 
all  photographs  to  represent  coloured  objects  untruly 
in  their  relative  luminosity.  It  was  an  old  complaint 
that  blues  photographed  like  white,  and  reds  came  out 
like  black.  The  first  steps  towards  remedying  this  arose 
in  the  discoveries  of  Vogel  and  of  Abney  that  by  staining 
the  film  or  by  giving  to  it  in  its  preparation  as  an 


iv  THE  INVISIBLE  SPECTRUM  183 

emulsion  a  fine  granulation,  its  sensitiveness  toward  the 
longer  visible  waves  might  be  increased.  Thus  were 
introduced  the  orthochromatic  plates  which  gave  as 
photographs  a  more  accurate  representation  in  black 
and  white  of  the  relative  luminosities  of  objects ;  the 
ideal  orthochromatic  plate  being  one  which  should  have 
the  same  relative  sensitiveness  toward  the  light  of  each 
part  of  the  visible  spectrum  as  our  eyes  have.  Even 
before  these  discoveries  the  theory  of  the  trichroic 
method  of  reproducing  colour  by  photography  had  been 
enunciated  by  Clerk  Maxwell.1  In  the  theory  of  colour- 
vision  originated  by  Thomas  Young,  all  colour-sensations 
are  referred  to  three  simple  or  primary  colour-sensations, 
and  it  can  be  shown  that  no  more  than  three2  are 
needed  to  account  for  the  various  phenomena  of  colour- 
vision.  These  three  primary  sensations  are  the  sensation 
of  red,  the  sensation  of  green  (a  full  green  inclining  to 
yellowish-green),  and  the  sensation  of  blue-violet  (a  violet 
inclining  toward  blue).  A  red  light  stimulates  but  one 
of  these  sensations  in  the  nerves  of  the  eye.  A  yellow 
light  stimulates  two,  namely,  red  and  green,  and  is  not 
therefore  itself  a  primary  sensation.  Now  if  we  could 
take  three  photographs  of  an  object,  each  photograph 
corresponding  only  to  the  light  of  each  primary  sort, 
and  if  we  could  then  illuminate  each  photograph  with 
its  own  kind  of  light  and  superpose  them,  we  ought  to 
get  a  reproduction  of  the  natural  colours  of  the  object. 
That,  briefly,  is  the  three-colour  method. 

1  Discourse  at  Royal  Institution,  1 7th  May  1861. 

2  The  reader  should  consult  Captain  Abney's  treatise  on  Colour- 
vision. 


1 84  LIGHT  LECT. 

The  true  photography  of  colours  was  only  discovered 
a  year  or  two  ago  by  Professor  Lippmann,  whose 
exceedingly  precious  and  beautiful  results  are  individual 
pictures,  incapable  of  being  multiplied  or  reproduced. 
By  placing  at  the  back  of  the  transparent  sensitive  film 
a  mirror  of  mercury,  each  train  of  waves  is  reflected 
back  during  the  exposure;  and  where  the  reflected 
waves  meet  the  advancing  waves  of  the  train  they  set 
up  stationary  nodes  that  are  spaced  out  through  the 
thickness  of  the  film  at  distances  apart  corresponding 
to  the  exact  wave-lengths  of  the  various  lights.  At 
these  nodes  the  chemical  action  takes  place,  and  pro- 
duces a  permanent  picture  which,  when  viewed  by 
reflected  light,  shows  all  the  natural  colours  of  the  object 
that  has  been  photographed.  I  am,  by  the  kindness  of 
my  colleague,  Professor  Meldola,  able  to  show  here,  and 
to  project  on  the  screen,  a  Lippmann  photograph  of  the 
spectrum  in  which  all  the  colours  show  in  their  natural 
tints.  More  recently  Professor  Lippmann  has  shown  in 
this  theatre  the  remarkable  colour-pictures  which  he  has 
produced  of  landscapes,  still-life  subjects,  and  even  of 
the  human  figure.1 

Returning  to  the  three-colour  method  of  registering 
and  reproducing  by  photography  the  natural  colours  of 

1  Since  the  delivery  of  these  lectures  two  new  processes  of 
colour-photography  have  been  announced.  In  one  of  these  by 
M.  Chassagnes  certain  chemicals  are  said  to  be  used  to  treat  the 
photographic  films,  by  virtue  of  which  they  become  capable  of 
absorbing  pigments  at  those  parts  of  the  picture  which  have  been 
impressed  by  light  of  the  corresponding  tint.  Another  process 
by  Mr.  Benetto  produces  coloured  transparencies  by  direct  photo- 
graphy. 


iv  THE  INVISIBLE  SPECTRUM  185 

objects,  I  am  happy  in  conclusion  to  be  able  by  the 
kindness  of  Mr.  Ives  to  show  you  the  remarkable  results 
which  he  has  attained  with  his  photochromoscope. 
Starting  from  Young's  theory  of  the  three  primary 
sensations,  Mr.  Ives  sought  to  construct  colour  filters 
which  should  transmit  for  each  of  the  three  primaries 
all  those  waves  of  the  spectrum  which  excite  that 
sensation,  and  in  proportion  to  their  power  of  exciting 
that  sensation  in  the  eye.  Thus  the  filter  for  red  should 
transmit  not  simply  red  light,  but  should  transmit  all 
those  waves  of  whatever  colour  that  are  competent  to 
excite  the  red  sensation,  but  transmit  them  only  in 
proportion  as  they  are  competent  to  excite  the  red 
sensation.  To  select  the  proper  tints  as  colour  filters 
is  a  matter  of  no  small  skill  and  experience.  Through 
three  such  screens — one  for  red,  one  for  green,  one  for 
blue -violet — three  photographs  (negatives)  are  taken 
simultaneously  side  by  side  upon  a  single  orthochro- 
matic  plate.  From  these  three  negatives  (which  are  of 
course  colourless  themselves)  three  positives  are  printed. 
These  also  are  colourless,  but  they  show  differences 
according  to  the  colours  of  the  different  parts  of  the 
object  photographed.  Fig.  105  is  a  triple  chromogram 
of  a  butterfly.  Its  wings  beside,  having  definite  patches 
of  red  and  white  on  a  black  background,  have  all  over 
them  a  beautiful  sheen  of  brilliant  blue.  The  upper- 
most image  of  the  three  is  that  which  is  to  be  placed  in 
the  blue-violet  light.  The  second  figure  is  that  for  the 
green  light,  while  the  lowest  is  for  the  red  light.  Those 
parts  which  are  to  show  as  white,  when  combined,  are 
white  in  all  three  images.  Each  image  is  itself,  like  the 


i86 


LIGHT 


LECT. 


printed  cut,  colourless;   a  mere  black  and  white  tran- 
script on  glass. 

Now  let  these  three  colourless  pictures  be  placed  in  an 
instrument  so  arranged  that  blue-violet  light  falls  through 


FIG.  105. 

x.  To  be  illuminated  by  Blue-Violet  Light. 
2.  To  be  illuminated  by  Green  Light.  3.  To  be  illuminated  by  Red  Light. 

the  first,  green  through  the  second,  and  red  through 
the  third  of  these  separate  photographs,  and  let  them 
then  be  recombined  by  suitable  mirrors  so  that  the  eye 
shall  view  them  simultaneously,  the  primary  colours 


IV 


THE  INVISIBLE  SPECTRUM 


187 


FIG.  106. 


will  recombine  and  give  the  object  in  all  the  glory  of 
the  natural  tints. 

The  instrument  (Photochromoscope  or  Kromskop) 
which  Mr.  Ives  has  designed  for  recombining  these  triple 
photographs  stands  upon 
the  table.  Fig.  106  gives 
a  diagram1  of  its  construc- 
tion. Mr.  Ives  has  also 
brought  a  lantern  photo- 
chromoscope,  by  means  of 
which  he  will  now  project 
on  the  screen  a  few  of  those 
beautiful  photographs. 
The  lantern  itself  has  three 
nozzles,  through  which  the 

red,  the  green,  and  the  blue-violet  pictures  are  separately 
projected  on  the  screen,  and  by  their  overlapping  give 
the  colour-combinations.  He  first  shows  us  separately 
the  three-coloured  disks  or  circles  of  light,  red,  green, 
and  blue-violet.  Then  he  moves  the  nozzles  so  that 

1  A,  B,  and  C  are  red,  blue-violet,  and  green  glasses  against 
which  the  three  corresponding  transparent  photograms  are  respect- 
ively placed.  Two  of  the  pictures  at  A  and  B  are  illuminated 
directly  by  light  from  above,  the  third  C  is  illuminated  by  an  oblique 
reflector.  The  red  picture  is  viewed  by  rays  which  are  reflected  at 
the  front  surface  of  D  an  oblique  transparent  glass  sheet.  The 
blue-violet  picture  at  B  sends  its  light  down  upon  another  oblique 
transparent  sheet  at  E,  which  reflects  it  through  the  sheet  D 
to  the  eye.  The  green  picture  at  C  is  viewed  through  both  the 
transparent  reflectors  D  and  E.  The  lens  F  collects  the  rays  for 
the  eye,  which  thus  views  the  three  pictures  as  if  they  were  super- 
posed and  at  equal  distance  away.  The  instrument  is  made 
binocular,  so  the  eyes  see  as  it  were  a  single  image  in  its  natural 
colours,  and  in  solid  relief. 


i88 


LIGHT 


LECT. 


FIG.  107. 


the  three  disks  partially  overlap  as  in  Fig.  107.     Where 
red   and  green  mix  they  give  us  yellow;    where  green 

and  blue  -  violet  overlap 
they  give  us  a  peacock 
blue;  where  blue -violet 
and  red  overlap  they  give 
us  purple  ;  where  all  three 

\x  /WHITE\  /B'LUE  ^   overlap  at  the  centre  they 
PM  L- rin^T  I  give  us  white.     Note  how 

the  tint  produced  by  the 
overlap  of  two  gives  us  the 
complementary  to  the 
third.  Thus  the  yellow 
is  the  complementary  to 
the  blue-violet ;  the  peacock  is  complementary  to  the 
red ;  and  the  purple  is  of  a  tint  complementary  to  the 
green. 

Those  three  photographs  of  the  butterfly  are  now 
introduced  into  the  chromoscope -lantern,  and  are 
brought  to  accurate  superposition.  The  blue  shimmer 
on  the  insect's  wings  is  shown  with  marvellous  fidelity. 
No  painter  could  hope  to  produce  by  pigments  such  a 
natural  picture.  Here  is  a  photograph  of  a  basket  of  fruit. 
Note  the  yellow  lemon.  On  examining  the  separate 
colour-pictures  one  sees  that  this  yellow  is  made  up  of  the 
red  and  green  lights  mixed.  Here  is  a  gorgeous  bouquet 
of  flowers.  The  colours  are  superb.  Here  is  a  cigar- 
box  showing  the  natural  brown  tint  of  the  wood ;  and 
beside  it  a  piece  of  cloisonne  enamel,  with  all  the  delicate 
shades  of  dull  tints  in  their  due  relations.  And  lastly, 
here  is  a  box  of  sweetmeats,  so  naturally  photographed 


iv  THE  INVISIBLE  SPECTRUM  189 

that  one  feels  them  to  be  really  edible.  After  that  we 
need  no  further  proof  that  by  the  proper  selection  of 
the  primary  tints  the  dream  has  at  last  been  realised  of 
registering  and  reproducing  by  photography  the  colours 
of  natural  objects. 


APPENDIX    TO    LECTURE    IV 

TABLE    OF    WAVE-LENGTHS    AND    FREQUENCIES 


Name  of  Line. 

Element. 

Wave-length. 

Frequency 
(billions 
per  second). 

Micro-cen- 
timetres. 

Millionths 
of  inch. 

(Rubens  and  Nichols'  longest 

2400 

944 

12-5  (x  io12) 

waves) 

(Langley's  longest  waves) 

1500 

592 

20 

(Paschen's  longest  waves) 

945 

370 

31-7 

|i         j 

... 

270 

106-24 

III 

$2 

I24 

4873 

242 

^1 

1  2O 

47-25 

250 

{ 

-  B      -04 
09-865 

3536 
35  '35 

3337 

X4 

... 

88-06I 

340-8 

X3 

86-614 

34-i 

346-2 

X2 

85-418 

33  '63 

35I-3 

X: 

^7        33-44 

3533 

Z 

82  -04      32-34 

364-5 

A 

6 

75  "94- 

,#9-28 

395-2 

B 

0 

68-674 

27-03 

C 

H 

65*630 

25-83 

457-2 

Dj 

N2 

58-961 

23-21 

508-8 

D2 

N2 

58-902 

23-18 

509-1 

D3 

He 

58-760 

23-13 

Fe 

52705 

20-78 

569-2 

Ca 

52-704 

20-78 

569-2 

E2 

Fe 

52-697 

20-74 

569'3 

Mg 

51-838 

20-40 

578-9 

b\ 

Mg 

51729 

20-36 

580-0 

h        { 

Fe 
Fe 

51-692 
51-691 

20-351 
20-350 

580-4 
580-4 

APP. 


TABLE  OF  WAVE-LENGTHS,  ETC. 


TABLE   OF    WAVE-LENGTHS   AND    FREQUENCIES — Contimied 


Name  of  Line. 

Element. 

Wave-length. 

Frequency 
(billions 
per  second). 

Micro-cen- 
timetres. 

Millionths 
of  inch. 

r 

Fe 

5I-677 

20-306 

580-5 

'*           [ 

Mg 

5I>675 

20-305 

580-5 

F 

H 

48-615 

19-14 

6I7-I 

Gl 

Fe 

43-081 

16-96 

696-3 

-\ 

Ca 

43-079 

16-95 

696-4 

h 

H 

41-018 

16*17 

73  1  "3 

H 

Ca 

39-686 

15-63 

756-0 

K 

Ca 

39-338 

15-48 

762-7 

L 

Fe 

38-206 

15-04 

785-1 

M        ( 

Fe 

37-278 

14-676 

804-6 

I 

Fe 

37-271 

14-673 

804-9 

N 

Fe 

35-8I3 

14-09 

8377 

O 

Fe 

871-8 

P 

Fe 

33-6I3 

I3-23 

892-6 

Q 

Fe 

32-869 

12-94 

912-6 

R      { 

Ca 

31-814 

12-52 

942-9 

Ca 

31-794 

I2-5I 

943-5 

r 

Fe 

3  1  -446 

12-38 

954*1 

Si          f 

Fe 

31-008 

12-207 

967-4 

1 

Fe 

•004 

1  2  '2O6 

967-6 

I 

Fe 

31-001 

I2-2O5 

9677 

j 

Fe 

30-477 

11-99 

984-5 

T        j 

Fe 

30-212 

1  1  -894 

993-0 

I 

Fe 

•30-207 

1  1  -892 

993'3 

/ 

29*945 

11-79 

1002-0 

U 

29-480 

II  '60 

IOI7-6 

(Miller's  limit,  photogi  xjhic) 

20-2 

7^5 

1485-1 

(Stokes'  limit,  fluorescent) 

18-5 

7-28 

I62I-6 

(Schumann's  highest 

10 

3-93 

30OO 

frequency) 

LECTURE  V 

THE    INVISIBLE   SPECTRUM    (iNFRA-RED    PART) 

How  to  sift  out  the  invisible  infra-red  light  from  the  visible  light — 
Experiments  on  the  absorption  and  transmission  of  invisible 
infra-red  light — It  is  cut  off  by  transparent  glass,  but  trans- 
mitted by  opaque  ebonite — Use  of  radiometer — Use  of  thermo- 
pile and  bolometer — "Heat-indicating"  paint — Experiments 
on  the  reflexion,  refraction  and  polarisation  of  invisible  infra- 
red Hght — Discovery  by  Hertz  of  propagation  of  electric  waves 
— Hertzian  waves  are  really  gigantic  waves  of  invisible 
light — Experiments  on  the  properties  of  Hertzian  waves  ;  their 
reflexion,  refraction  and  polarisation  —  Inference  that  all 
1  light-waves,  visible  and  invisible,  are  really  electric  waves  of 
different  sizes. 

TODAY  we  deal  with  those  waves  of  invisible  light 
which  lie  beyond  the  red  end  of  the  spectrum.  They 
are  invisible  to  us  because  their  wave-lengths  are  longer 
than  any  to  which  the  nerve-structures  of  our  eyes  are 
sensitive ;  or,  to  put  it  in  the  inverse  way,  because  their 
vibrations  are  of  a  frequency  lower  than  any  within  our 
range  of  optical  perception. 

Just  as  the  ultra-violet  waves  have  a  shorter 
wave-length  and  a  higher  frequency  than  the  visible 
waves,  and  have  to  be  detected  by  their  chemical, 
luminescent,  and  diselectrifying  effects,  so  the  infra-red 


LECT.  v  THE  INVISIBLE  SPECTRUM  193 

waves  of  larger  wave-length  and  lower  frequency  have  to 
be  detected  and  investigated  by  other  physical  effects 
than  that  of  sight.  The  chief  physical  effect  produced 
by  these  long  infra-red  waves  is  that  of  warming  the 
things  upon  which  they  fall.  For  this  reason  they  are 
sometimes  called  the  calorific  waves ;  and  the  invisible 
light  of  this  kind  is  sometimes  *  called  "radiant  heat." 

But  if,  as  I  shall  have  to  show  you,  this  so-called 
radiant  heat  possesses  (save  in  respect  of  visibility)  all 
the  physical  properties  of  light ;  if  we  can  reflect  it,  and 
refract  it,  disperse  it,  diffract  it,  and  polarise  it,  then  we 
are  logically  compelled  to  admit  that  it  is  really  a  kind 
of  light. 

In  brief,  the  spectrum  extends  both  ways  beyond  the 
visible  part ;  beyond  the  violet  are  the  chemical  waves, 
and  below  the  red  are  the  heat  waves. 

If  you  find,  as  every  one  finds,  that  the  light  from  the 
sun  or  from  a  flame  warms  the  things  on  which  it 
shines,  it  is  natural  to  ask  which  of  all  the  waves  mixed 
up  together  in  the  beam  give  the  warmth.  To  answer 
that  question  let  us  have  recourse  to  the  test  of  a 
carefully  considered  experiment.  First  let  us  spread, 
out  the  rays  into  a  spectrum,  and  then  explore  which 
part  has  the  greatest  warming  effect. 

The  first  explorations  of  the  spectrum  were  made  by 
putting  into  the  different  parts  of  the  spectrum  the 
bulb  (blackened)  of  a  thermometer.  This  showed 

1  Another  term  used  by  some  writers  is  "the  radiation."  This 
use  of  the  term  is  to  be  deprecated  ;  for  the  word  radiation  ought 
not  to  be  used  in  two  senses.  If  it  is  rightly  used  to  mean  the  act 
of  radiating,  then  some  other  term  ought  to  be  used  to  denote  that 
which  is  radiated,  namely,  the  waves. 

O 


194  LIGHT  LECT. 

that  the  heating  effect  is  mainly  at  and  beyond  the  red 
end  of  the  spectrum. 

The  spectrum  which  is  once  again  thrown  on  the 
screen  (Fig.  108)  is  produced  as  on  previous  occasions 
by  employing  in  the  lantern  an  electric  arc-lamp,  in 
front  of  which  is  placed  a  slit,  a  lens  to  focus  an  image 
of  the  slit,  and  a  prism  to  disperse  the  mixed  waves 
into  their  proper  places  according  to  their  wave-length. 

Our  spectrum  to-day  is  neither  so  brilliant  nor  so 
extended  as  you  have  seen  it  on  former  occasions ;  the 
cause  for  this  circumstance  being  that  (for  reasons  you 
will  presently  appreciate)  we  are  obliged  to  abandon  the 
use  of  glass  lenses  and  glass  prisms,  and  substitute  lens 
and  prism  of  rock-salt.  This  material  is  less  refractive 
and  less  dispersive,  hence  the  narrowness  of  the  rainbow- 
coloured  band. 

And  now  we  have  to  make  good  by  experiment  the 
proposition  that  I  have  advanced  that  the  heating  effect 
is  due  to  the  longest  waves — those  at  the  red  end  and 
beyond  the  red  end  of  the  spectrum. 

But  as  an  ordinary  thermometer  would  not  be  con- 
venient I  adopt  another  method,  using  instead  a  sort  of 
electric  thermometer — the  thermopile.  If  you  want  to 
know  all  about  this  instrument,  you  must  refer  to 
treatises  on  electricity.  All  I  need  say  now  about  it  is 
that  it  is  an  apparatus,  Fig.  109,  which  is  exceedingly 
sensitive  to  heat,  and  which,  when  the  face  of  it  is 
warmed,  generates  an  electric  current.  The  electric 
current  is  led  into  a  galvanometer  which  reflects  a  spot 
of  light  upon  the  scale  against  the  wall.  So,  you  may 
take  it  that  that  spot  of  light  will  indicate  by  its  position 


THE  INVISIBLE  SPECTRUM 


IQ5 


whether  the  face  of  the  thermopil 


e  is  warmer  or  colder 


196 


LIGHT 


LECf. 


spot 


than  the  air  of  the  room.      If   it  is  warmer  the 
will  move  to  the  right ;  if  colder,  to  the  left. 

The  spectrum  now  falls  upon  a  small  brass  screen 
with  a  slit  in  it,  behind  which  is  the  thermopile ;  and  at 
present  the  part  of  the  spectrum  that  enters  through  the 
slit  and  falls  on  the  face  of  the  pile  is  the  ultra-violet 
part.  The  spot  of  light  is  still  at  zero,  showing  that  the 
ultra-violet  light  does  not  appreciably  warm  the  face  of 


FIG. 


the  pile.  I  now  explore  the  spectrum  by  pushing  the 
thermopile  gently  along.  The  slit  now  lies  in  the  violet 
— yet  there  is  no  heating  effect.  The  blue,  the  peacock, 
the  green,  and  the  yellow  are  successively  explored — 
yet  the  spot  of  light  remains  at  the  zero.  These  waves 
do  not  produce  appreciable  heating.  Another  move 
forward,  and  the  orange  waves  enter  the  slit  and  fall  on 
the  face  of  the  pile — the  spot  begins  to  move.  The 
orange  waves  warm  slightly.  I  push  on  into  the  red, 


v  THE  INVISIBLE  SPECTRUM  197 

and  the  spot  moves  gently  across  about  a  quarter  of 
the  scale.  -  Red  waves  heat  more  than  orange  ones. 
Pushing  on  beyond  the  end  of  the  visible  red  (Fig.  108) 
the  effect  increases.  At  a  point  about  as  far  beyond 
the  end  of  the  red  as  the  red  is  beyond  the  green  of  the 
spectrum,  the  heating  effect  is  much  greater — the  spot 
flies  across  the  scale.  Clearly  the  waves  which  have  the 
greatest  calorific  power  are  those  some  little  way  in  the 
invisible  infra-red  region :  or  in  other  words  the  waves 
that  heat  most  are  waves  having  a  wave-length  somewhat 
greater  than  that  of  the  largest  waves  of  the  visiHle 
spectrum.  Taking  the  size  of  the  extreme  red  waves  at 
32  millionths  of  an  inch,  we  may  put  down  these  more 
powerful  invisible  waves  as  about  40  to  45  millionths 
of  an  inch  in  length. 

The  invisible  infra-red  spectrum  has  often  been 
explored,  and  by  many  explorers.  Langley,  using  a 
different  electric  instrument  of  his  own  invention, 
termed  a  bolometer^  has  succeeded  in  observing  waves 
whose  length  was  592  millionths  of  an  inch,  or  which 
have  a  wave-length  twenty  times  as  great  as  those 
of  red  light.  Professor  Rubens  has  independently 
measured  infra-red  waves  as  large  as  0*002400  centi- 
metre, or  about  944  millionths  of  an  inch  in  length. 
Hence,  if  set  down  in  a  scale  of  wave-lengths,  the 
infra-red  spectrum  stretches  out  to  about  fifty  times 
the  extent  of  the  visible  spectrum.  In  the  language 
adopted  for  describing  musical  intervals,  while  the 
range  of  visible  light  is  about  one  octave  (the  extreme 
violet  having  about  double  the  frequency  of  the  ex- 
treme red),  the  infra-red  waves  are  known  to  go  down 


198  LIGHT  LECT. 

more  than  five  octaves  below,  and  the  ultra-violet  waves 
ascend  to  about  two  octaves  above  the  visible  kinds  of 
waves  (see  the  Table  on  pp.  190,  191). 

Our  thermopile  has  been  placed  at  that  part  of  the 
spectrum,  a  little  beyond  the  end  of  the  visible  red, 
where  we  found  the  greatest  heating  effect.  To 
increase  the  effect  somewhat,  I  will  open  the  slit  a 
little,  thus  permitting  a  larger  amount  of  these  longer 
waves  to  fall  upon  the  face  of  the  instrument.  Having  \ 
thus  adjusted  our  arrangements  so  as  to  be  sensitive  to 
the  heat-waves,  we  will  try  an  experiment  or  two  to  find 
whether  these  longer  waves  which  produce  the  heating 
effect  are  able  to  penetrate  through  the  various  materials 
which  we  have  tried  for  ordinary  light.  In  the  first 
place,  take  a  piece  of  window-glass,  and  try  whether  the 
heat-waves  will  pass  through  it.  On  interposing  it  in 
front  of  the  slit  we  notice,  by  the  indication  given  by 
the  galvanometer  and  thermopile,  that  though  it  cuts 
off  much  of  the  heat  it  does  not  cut  it  off  entirely. 
Substituting  a  piece  of  red  glass,  we  find  that  it  also 
cuts  off  some  of  the  effect,  but  a  blue  glass  cuts  it  off 
much  more.  Now  I  take  a  piece  of  flint  glass,  which 
contains  lead :  you  note  that  it  cuts  off  the  waves  much 
more.  Here  is  a  slice  of  quartz  crystal ;  it  does  not  cut 
off  the  effect  as  much  as  the  glass  did.  Again,  here  is 
a  slice  of  calc-spar  of  the  same  thickness.  It  cuts  off 
the  heat-waves  more  completely  than  any  of  the  mate- 
rials I  have  yet  tried.  Lastly,  here  is  a  slice,  also  of 
the  same  thickness,  of  rock-salt ;  that  is  to  say,  a  slice 
of  a  big  crystal  of  common  salt  sawn  off  and  polished. 
The  rock-salt  hardly  cuts  off  the  heat-waves  at  all. 


THE  INVISIBLE  SPECTRUM 


199 


Here  then  are  four  substances — glass,  quartz,  calc-spar, 
and  rock-salt — all  transparent  alike  to  ordinary  light. 
Quartz,  as  we  saw  in  the  last  lecture,  is  exceedingly 
transparent  to  the  ultra-violet  kind  of  invisible  light ; 
that  is  to  say,  to  the  shortest  waves.  But  to-day  we 
prove  that  rock-salt  is  the  one  that  is  most  transparent 
to  the  infra-red  kind  of  invisible  light.  Naturally,  seeing 
that  this  fact  was  discovered  half  a  century  ago,  we  now 
apply  the  discovery  in  the  construction  of  our  apparatus. 
The  lenses  and  the  prism  I  am  using  to-day  for  these 
experiments  are  made  neither  of  glass  nor  of  quartz, 
but  of  rock-salt.  And  as  rock-salt  possesses  a  very 
poor  dispersive  power  for  the  visible  kinds  of  waves,  it 
produces,  as  you  have  seen,  but 
a  poor  spectrum  of  colours  as 
compared  with  the  spectrum  that 
would  be  produced  by  the  use 
of  a  prism  of  glass  or  quartz  of 
the  same  size. 

We  might  have  used  as  an 
exploring  apparatus,  instead  of 
our  thermopile,  another  instru- 
ment, the  radiometer  (Fig.  no). 
Rather  more  than  twenty  years 
ago  the  celebrated  chemist 
Crookes  made  the  discovery  that 
when  light  falls  on  movable 
things,  such  as  the  vanes  of  a 
lightly  -  poised  mill,  in  a  glass 
bulb  from  which  the  air  has 
been  mostly  exhausted,  so  as  to  leave  a  fairly  perfect 


200  LIGHT  LECT. 

vacuum,  the  vanes  of  the  mill  are  driven  round. 
Apparently  the  blackened  vane  of  the  mill  tends  to 
retreat  from  the  light.  Why,  we  must  presently  con- 
sider. My  present  point  is  that  it  is  possible  to  use 
this  apparent  repulsion  to  measure  the  intensity  of  the 
radiation  that  falls  upon  the  instrument.  Place  the 
radiometer  in  one  part  of  the  spectrum ;  it  turns  round 
slowly.  Move  it  on  into  the  red  end  ;  it  spins  more 
quickly.  But  the  effect  is  found  to  depend  not  merely 
upon  the  kind  of  waves,  but  also  to  some  extent  upon 
the  nature  of  the  surface  of  the  vanes,  and  upon  the 
degree  of  vacuum  in  the  bulb.  Some  radiometers 
revolve  most  rapidly  in  the  bright  part  of  the  spectrum 
to  which  our  eyes  are  sensitive. 

Whether  we  explore  the  spectrum  with  a  thermo- 
meter, as  Sir  William  Herschel  did,  or  with  a  thermo- 
pile, or  a  bolometer,  or  a  radiometer,  we  find  that  it 
consists  of  waves  spread  out  in  different  directions,  and 
that  the  different  waves  have  different  heating  powers. 
And  yet  all  these  different  waves,  with  their  different 
powers,  are  emitted  at  one  and  the  same  time  from  the 
same  source.  If  the  thing  that  is  heated  is  insuffi- 
ciently heated  it  will  not  shine — that  we  all  know. 
But  even  if  not  hot  enough  to  shine  visibly  it  will  still 
emit  some  invisible  waves.  When  you  begin  to  warm 
a  substance,  at  first  it  gives  out  only  a  few  waves  of 
very  long  wave-length.  As  you  heat  it  more  it  gives 
out  more  of  these  heat-waves,  and  along  with  these 
heat-waves  it  also  gives  out  some  visible  waves  of  shorter 
wave-length.  If  heated  still  hotter,  so  as  to  be  white- 
hot,  it  gives  out  not  only  heat-waves  of  all  sorts,  but 


v  THE  INVISIBLE  SPECTRUM  201 

visible- waves  from  red  to  violet,  and  also  with  ultra- 
violet waves,  all  mixed  up  together. 

In  the  next  experiment  we  will  employ  a  dark  source 
of  waves.  In  short,  we  will  test  the  waves  that  are 
emitted  from  a  vessel  of  hot  water.  Here  is  a  beaker- 
glass  which  I  fill  with  boiling  water  from  the  kettle. 
You  would  not  see  that  in  the  dark,  would  you  ?  In  a 
perfectly  dark  room  it  would  not  give  out  any  of  those 
waves  to  which  your  eyes  are  sensitive.  But  if  you  were 
to  hold  your  hand  a  few  inches  away  from  it  you  would 
feel  a  gentle  warmth  radiating  from  it.  You  can  feel 
with  the  nerves  of  your  hand  that  which  the  nerves  of 
your  eyes  do  not  perceive,  namely,  the  long  waves  or 
calorific  radiations.  But  these  long  waves  warm  all 
things  on  which  they  fall.  They  do  not,  however,  warm 
them  all  equally.  The  fact  can  be  established  in  many 
ways.  Black  and  dark  substances  absorb  the  waves 
that  fall  upon  them.  Bright  and  shiny  bodies  reflect 
most  of  the  waves.  What  becomes  of  the  waves  that 
fall  on  black  and  dark  bodies  ?  Their  energy  is  not  lost ; 
it  is  transmuted  into  sensible  heat.  Instead  of  wave- 
motions  in  the  free  space,  we  have  molecular  vibrations 
in  the  substance.  The  bright  surface,  such  as  polished 
metal,  upon  which  the  waves  may  fall,  is  not  warmed 
by  them,  for  the  waves  as  they  meet  the  surface  are 
not  broken  up,  but  simply  start  off  again  in  some  new 
direction.  Whenever  waves  break  on  a  surface,  and 
are  destroyed  or  absorbed  in  the  action  of  breaking,  the 
result  is  heat.  The  so-called  heat-waves,  or  infra-red 
waves,  are  not  themselves  hot.  They  do  not  heat  the 
medium  through  which  they  travel  as  waves.  But  they 


202 


LIGHT 


LECT. 


B 


are  readily  absorbed  by  the  things  they  fall  upon,  and, 
being  absorbed,  they  warm  that  on  which  they  fall.  A 
black  surface  is  one  which  absorbs  both  the  invisible 
and  the  visible  waves.  It  heats  more  readily  than  a 
white  or  a  bright  surface. 

Now  here  is  a  peculiar  thermometer  (Fig.  in), 
having  two  bulbs,  full  of  air,  joined  together  by  a  bent 
tube,  containing  a  little  coloured 
liquid  to  serve  as  an  index :  it 
stands  up  to  the  height  marked 
a.  If  I  put  my  hand  on  either 
bulb,  and  so  warm  the  air  in- 
side it,  the  expansion  of  the  air 
will  depress  the  liquid  in  the 
tube  below  the  bulb  that  is 
warmed.  Were  both  warmed 
equally  the  liquid  would  not 
move.  So  this  apparatus  will 
indicate  a  difference  of  tempera- 
ture, and  is  therefore  called  a  differential  thermometer. 
Next,  note  that  one  of  the  bulbs,  B,  has  been  painted  dull 
black,  the  other,  G,  has  been  gilt  with  gold-leaf.  I  am 
going  to  put  the  beaker  of  hot  water  exactly  in  the  middle 
between  the  two  bulbs,  where  it  can  radiate  equally  to 
both  of  them.  The  gilt  bulb,  having  a  bright  surface, 
reflects  the  waves,  and  is  scarcely  warmed  at  all  by 
them.  But  the  black  bulb,  having  a  more  absorptive 
surface,  will  be  warmed  more  than  the  bright  gilt  one ; 
and  you  will  see  the  indicating  liquid  fall  in  the  tube 
below  B  and  rise  in  the  tube  below  G.  Had  we 
employed  a  beaker  half  blackened  and  half  gilt,  we 


FIG. 


THE  INVISIBLE  SPECTRUM 


203 


could  have  readily  demonstrated  another  point,  namely, 
that  a  hot  black  surface  radiates  out  the  heat-waves 
more  readily  than  a  hot  bright  surface  does. 

Now  let  us  pass  on  to  another  experiment  in  which 
we  again  employ  the  thermopile.  Here  is  a  thermopile 
connected  by  wires  to  the  galvanometer,  with  its  re- 
flected spot  of  light  on  the  wall.  It  is  arranged  so  that 
if  the  face  of  the  thermopile  is  warmed  the  spot  will 
move  to  the  right  and  indicate  to  you  the  circumstance. 
In  front  of  the  conical  mouth-piece  of  the  thermopile 


FIG. 


stands  an  ordinary  Bunsen  burner,  which,  as  you  know, 
is  a  gas  jet  having  openings  at  the  foot  to  let  atmospheric 
air  mix  with  the  gas.  It  gives  a  smokeless  blue  flame 
very  different  from  the  ordinary  bright  flame  of  gas. 
Though  very  hot,  this  flame  radiates  out  but  little  light, 
neither  does  it,  as  a  matter  of  fact,  radiate  off  much  heat. 
True,  a  column  of  hot  air  ascends  from  it  straight  up. 
But  that  is  not  what  I  am  thinking  of.  The  question 
is,  Is  it  sending  out  heat  sideways?  Well,  we  can  try. 
Opening  the  metallic  shutter  that  closes  the  mouth- 
piece of  the  thermopile  I  let  the  light  and  heat,  such  as 
they  are,  radiate  from  the  flame  upon  the  face  of  the 


204  LIGHT  LECT. 

pile.  At  once  the  spot  of  light  on  the  wall  moves  off 
to  the  right,  showing  that  there  are  at  any  rate  some 
waves  present  that  can  heat  the  pile.  If  I  interpose  for 
a  moment  a  sheet  of  glass  between  the  burner  and  the 
thermopile  the  spot  of  light  comes  back  almost  to  its 
zero,  showing  that  glass  screens  off  nearly  all  the  waves. 
I  remove  the  screen,  and  the  spot  goes  back  to  the 
right,  showing  that  the  heating  effect  has  recommenced. 
Now  comes  the  particular  point  of  the  experiment.  If 
I  stop  up  the  holes  at  the  foot  of  the  burner  where  the 
air  has  been  entering,  the  flame  will  at  once  burn 
brightly  as  an  ordinary  gas  flame.  The  combustion 
will,  as  a  matter  of  fact,  be  less  perfect,  for  the  flame 
will  be  sooty,  and  the  total  amount  of  heat  produced  in 
a  given  time  will  be  less,  because  of  the  imperfection  of 
the  combustion.  But  also  because  of  the  imperfection 
of  the  combustion  there  are  innumerable  solid  particles 
formed  in  the  flame  which  get  brilliantly  heated,  and 
emit  light.  They  are  better  radiators  of  waves  than 
the  gaseous  particles  of  the  pale  blue  flame,  and  they 
radiate  long  waves  better.  To  make  the  flame  shine 
thus,  I  have  but  to  stop  up  the  air-holes  with  my  finger 
and  thumb ;  and  instantly  the  spot  of  light  on  the  wall 
rushes  to  the  right,  even  beyond  the  end  of  the  scale, 
proving  that  the  bright  flame  radiates  more  heat-waves 
to  the  pile.  I  take  away  my  fingers,  air  is  readmitted,  the 
flame  relapses  to  its  former  pale  state,  and  the  spot  of  light 
settles  back  to  its  former  position.  Every  time  I  let  the 
flame  burn  brightly  it  radiates  more  waves  sideways. 

You  may  use  a  radiometer  instead  of  a  thermopile 
to  demonstrate  the  facts.     The  vanes  of  the  mill  turn 


v  THE  INVISIBLE  SPECTRUM  205 

fast  when  the  flame  is  bright,  and  more  slowly  when, 
by  admitting  air  to  the  flame,  you  improve  the  com- 
bustion. 

The  next  experiment  I  have  to  show  you  is  with  the 
same  thermopile,  only,  instead  of  shining  upon  it  with  a 
flame,  I  will  put  a  lump  of  ice  in  front  of  it.  The  spot 
of  light  on  the  wall  now  retreats,  right  beyond  the  zero 
mark,  to  the  left,  indicating  that  the  face  of  the  thermo- 
pile has  been  chilled.  Perhaps  you  will  say  that  this 
proves  that  the  ice  is  radiating  out  cold.  It  may  seem 
so.  But  that  which  is  really  occurring  is  this.  "  Cold  " 
is  a  relative  term  meaning  really  "less  hot."  All  things 
that  are  not  in  that  unattainable  state  of  absolute  zero 
of  temperature  are  more  or  less  hot ;  hot  things  more, 
cold  things  less.  And  everything  tends  to  radiate  its 
heat  away — the  hotter  it  is  the  greater  its  tendency. 
Ice  is  less  hot  than  the  other  things  in  this  room.  The 
ice  is  colder  than  the  thermopile.  The  thermopile 
itself  is  radiating  out  heat,  some  of  which  goes  to  the 
ice.  The  ice  is  also,  though  to  a  lesser  degree,  radiating 
out  heat.  Here  then  we  have  two  things,  a  thermopile 
which  is  warm,  and  ice  which  is  colder,  radiating  to  one 
another  unequally.  The  result  of  this  unequal  exchange 
is  that  the  thermopile  parts  with  more  heat  than  the  ice 
parts  with,  and  therefore  is  cooled.  But  the  effect  is 
the  same  as  if  the  cold  were  being  radiated. 

Now  let  us  go  to  an  experiment  that  I  believe  took 
its  origin  nearly  a  century  ago  in  this  Royal  Institution. 
In  the  Royal  Institution  the  emission  of  heat  and  the 
properties  of  heat-waves  have  ever  been  favourite  topics 
of  study.  The  founder  of  the  Royal  Institution,  Count 


206 


LIGHT 


LECT. 


Rumford,  himself  originated  many  experiments  on  the 
radiation  of  heat.  It  was  he  who  discovered  that  heat 
could  be  radiated  across  a  vacuum.  Sir  Humphry 
Davy,  while  Professor  here,  showed  a  most  beautiful 
experiment  in  which  heat-waves  were  reflected  from  one 
point  of  space  to  another  by  means  of  two  paraboloidal 
mirrors  of  silvered  metal.  That  experiment  I  propose 
to  repeat,  using  the  two  mirrors  which  are  believed  to 
be  the  actual  pair  used  by  Sir  Humphry,  and  often 
since  used  by  Professor  Tyndall. 

One  of  the  two  curved  mirrors  is  hung  mouth- 
downwards  at  a  height  of  some  fifteen  feet  above  the 
lecture-table.  The  other  stands 
mouth -up  wards  on  the  table 
exactly  beneath  the  first.  The 
upper  mirror  is  lowered  for  a 
moment.  A  red-hot  iron  ball 
is  slung  by  a  hook  in  the  focus 
of  the  mirror,  and  it  is  hoisted 
up  again  into  its  position  above 
the  table.  The  ball  being  at 
the  focus,  the  mirror  collects 
the  diverging  waves  and  reflects 
them  straight  down  in  a  parallel 
beam.  It  is  a  beam  of  invisible 
infra-red  waves,  accompanied 
by  a  few  waves  of  visible  ijfcd. 
This  beam  falls  upon  the  second 
mirror  (Fig.  113),  which  once  more  collects  them  and 
converges  them  to  a  focus,  F.  I  put  my  hand  at 
the  place  toward  which  the  waves  converge ;  it  is 


FIG.  113. 


v  THE  INVISIBLE  SPECTRUM  207 

intolerably  hot.  I  hold  in  the  focus  a  bit  of  black 
paper,  at  once  it  smokes,  and  kindles  into  visible  com- 
bustion. Other  things  can  be  lighted.  Here  is  a 
cigar.  Holding  it  in  the  focus  it  absorbs  enough  waves 
to  warm  it  up ;  and  the  ascending  wreath  of  smoke 
proves  that  it  has  been  kindled  by  the  reflected  and 
concentrated  waves. 

Our  experiment  has  proved  that  heat-waves  can  be 
reflected.  Our  earlier  experiments  with  the  rock-salt 
prism  proved  that  they  can  be  refracted.  Let  us  con- 
firm these  points  by  other  experiments. 

The  red-hot  ball,  fast  fading  into  dulness  as  it  parts 
with  its  store  of  energy,  has  now  been  placed  upon  a 
stand  on  the  table.  Taking  up  the  thermopile,  which 
did  such  useful  service  just  now,  we  will  see  what  it  can 
tell  us  about  that  red-hot  ball.  The  distance  between 
the  two  is  some  seven  or  eight  feet.  I  have,  however, 
only  to  turn  the  conical  mouth-piece  of  the  thermopile 
straight  toward  the  ball,  and  at  once  the  spot  of  light 
on  the  wall  indicates  that  the  thermopile  has  received 
some  of  the  radiation.  Waves  that  our  eyes  cannot  see, 
this  thermopile  can  see  if  only  we  turn  it  so  as  to  look 
straight  at  the  source  of  the  waves.  It  acts  as  a  kind 
of  eye  that  is  sensitive,  not  to  visible  light,  but  to  infra- 
red waves  of  invisible  light.  I  turn  the  aperture  of  the 
pile  on  one  side  so  that  none  of  the  heat-waves  can 
enter  it ;  the  spot  of  light  on  the  wall  settles  down  to 
its  zero.  Then,  taking  up  a  simple  piece  of  tin-plate 
to  serve  as  a  mirror  I  reflect  some  of  the  heat-waves 
from  the  iron  ball  back  into  the  mouth  of  the  thermo- 
pile. As  soon  as  the  mirror  is  set  at  the  proper  angle 


208  LIGHT  LECT. 

the   spot   moves  to   the   right,   showing  that  we   have 
reflected  some  of  the  heat-waves  into  the  pile. 

While  we  have  a  hot  ball — best  if  we  had  one  that 
was  brightly  red-hot — I  can  show  some  interesting  experi- 
ments which  turn  upon  the  employment  of  certain  heat- 
indicating  paints.1  Here  is  a  specimen  of  heat-indicating 
paint  of  a  scarlet  colour  that  turns  black  when  heated. 
Here  is  another  of  pale  yellow  tint  which  turns  red  even 
when  quite  gently  warmed.  Here  is  a  paper  screen 
mounted  in  a  convenient  frame.  The  front  is  painted 
over  with  yellow  heat-indicating  paint :  the  back  has  been 
blackened  that  it  may  the  better  absorb  the  heat-waves. 

1  These  heat-indicating  paints  are  double  iodides  of  mercury  with 
other  metals.  They  were  discovered  nearly  thirty  years  ago  by  Dr. 
Meusel.  The  scarlet  paint  that  turns  almost  black  at  about  87°  C.  is 
the  double  iodide  of  mercury  and  copper.  The  yellow  paint  which 
turns  red  at  about  45°  C.  is  the  double  iodide  of  mercury  and  silver. 
To  prepare  the  former,  a  solution  of  potassium  iodide  is  added  to  a 
solution  of  copper  sulphate  until  the  precipitate  is  redissolved,  when 
a  concentrated  solution  of  mercuric  chloride  is  added  precipitating 
the  red  double-iodide.  To  prepare  the  more  sensitive  yellow  paint, 
add  to  a  solution  of  silver  nitrate  a  solution  of  potassium  iodide 
until  the  precipitate  (silver  iodide)  redissolves.  To  this  solution  add 
a  concentrated  solution  of  mercuric  chloride  until  a  bright  yellow 
precipitate  is  formed.  The  precipitates  are  collected  on  filter  paper, 
and  should  be  washed  with  cold  water.  They  may  be  mixed 
with  very  dilute  gum-water  to  enable  them  to  be  used  as  paint. 
With  these  paints  many  interesting  experiments  can  be  performed  in 
illustration  of  the  propagation  of  heat  by  conduction  and  convexion 
as  well  as  by  radiation.  One  very  simple  experiment  is  worthy  of 
mention.  It  is  to  show  how  hot  water  will  float  on  cold  water.  A 
strip  of  paper  painted  with  the  yellow  paint  is  pasted  vertically 
against  the  outside  of  a  tall  glass  beaker.  This  is  half  filled  with 
cold  water.  A  floating  disk  of  wood  is  introduced  to  prevent  undue 
agitation,  and  then  the  beaker  is  filled  up  by  pouring  in  boiling  water 
out  of  a  kettle.  The  top  half  only  of  the  strip  of  paper  turns  red. 


v  THE  INVISIBLE  SPECTRUM  209 

Holding  it  a  little  way  from  the  hot  ball — an  ordinary 
coal  fire  answers  even  better — I  place  my  hand  between 
the  ball  and  the  screen,  against  the  back  of  it.  In  a 
few  seconds  the  screen  turns  red  all  over  except  where 
it  is  protected  by  my  hand,  of  which  a  shadow — a  sort 
of  heat-shadow — in  yellow  is  temporarily  photographed, 
or  rather  thermographed,  upon  the  screen.  As  the 
screen  cools  it  returns  to  its  former  yellow  tint. 

Here  is  another  screen  made  of  paper  painted  with 
scarlet  heat-indicating  paint.  The  back  has  been  gilt 
all  over,  and  then  on  the  gilt  surface  a  big  letter  S  has 
been  painted.  I  hold  this  with  its  gilt  back  to  the  hot 
ball ;  and  the  gilt  surface  reflects  away  most  of  the  heat- 
waves. Now  you  might  suppose  that  the  part  where 
black  paint  has  been  put  on  over  the  top  of  the  gold 
would  be  doubly  protected  against  heat.  But,  no  !  It 
.  absorbs  the  waves  and  grows  warm ;  and  the  heat  being 
conducted  through  the  gold  film  causes  the  scarlet  paint 
on  the  front  of  the  screen  to  turn  black.  The  letter 
painted  on  the  back  is  visible  on  the  front  of  the 
screen. 

Here  is  a  variation  upon  one  of  Professor  TyndalPs 
observations.  You  will  find  it  recorded  in  his  book1 
on  heat  how,  on  one  occasion  when  a  fire  broke  out  in 
a  street,  the  heat  radiated  across  the  street  from  the 
burning  house,  charred  the  window-frames  and  burned 
and  blistered  the  paint  on  the  sign-boards.  But  where 
the  number  of  the  house  stood  in  gilt  letters  on  the 
sign-board  the  mere  film  of  metal  had  reflected  away 
the  waves,  protecting  paint  and  wood  behind  it  from 

1  Heat  a  Mode  of  Motion ,  p.  263. 
P 


210  LIGHT  LECT. 

being  charred.  In  illustration,  here  is  a  blackened 
sheet  of  paper  upon  which  a  triangle  of  gold-leaf 
has  been  pasted.  The  other  surface  of  the  paper  has 
been  coated  with  the  scarlet  heat -indicating  paint. 
Exposing  it  to  the  radiation  of  the  hot  ball  you  see  how 
the  triangular  space  protected  by  the  gold-leaf  remains 
cool,  while  the  rest  absorbs  heat,  turning  the  scarlet  to 
black. 

You  will  probably  admit  that  we  have  now  plenty  of 
proofs  that  these  invisible  heat-waves  are  really  a  kind 
of  invisible  light :  that  the  difference  is  one  of  degree 
rather  than  of  kind.  Consider  yet  again  the  process  of 
incandescence  in  which  such  waves  are  emitted.  Heat  a 
body,  beginning  by  gently  warming  it.  At  first  its  particles 
vibrate  but  moderately ;  the  waves  they  send  out  into  the 
surrounding  ether  are  few  and  of  relatively  great  wave- 
length. As  you  warm  the  body  more  and  more  its 
particles  vibrate  more  actively,  they  jostle  together ;  it 
gives  out  more  waves  and  waves  of  shorter  length  and 
higher  frequency.  There  are  still  the  long  waves,  in  fact 
there  are  more  long  waves  than  before,  but  there  are  some 
shorter  waves  in  addition.  Heat  it  still  hotter.  The 
lower  kinds  of  waves  still  continue  to  be  emitted,  nay,  are 
emitted  more  copiously,  but  some  waves  of  a  still  higher 
kind  now  accompany  them.  Here  is  a  thin  platinum 
wire  stretched  between  two  supports.  By  leading  into  -it 
through  a  thicker  copper  wire  an  electric  current  I  can 
heat  it  as  little  or  as  much  as  I  choose.  It  is  now  warm, 
giving  out  a  few  dull  waves.  Increasing  the  current  its 
temperature  is  raised,  and  now  it  gives  out  much  more 
heat,  and  with  the  heat  a  few  waves  of  the  visible  red 


v  THE  INVISIBLE  SPECTRUM  211 

sort.  Every  solid  body  when  heated  shows  red  as  its 
first  colour  on  heating.  Never  is  the  first  glow1  of  a 
blue  or  yellow  hue.  Increasing  the  temperature  of  the 
wire  it  emits  orange  light  as  well  as  red,  and  looks  there- 
fore bright  red.  The  next  increase  brings  in  yellow 
along  with  orange  and  red  :  then  green  comes  in  to  join 
the  yellow,  orange,  and  red.  So  soon  as  the  wire  is 
heated  so  hot  as  to  give  out  all  the  different  visible 
kinds,  so  soon  we  call  the  state  a  white  heat.  But 
no  solid  ever  gets  blue  hot,  because  in  all  cases  the 
emission  begins  at  the  bottom  of  the  spectrum  with  red, 
the  other  colours  chiming  in  until  white  is  attained. 
Nor  is  white 2  attained  until  a  certain  proportion  of  the 
still  higher  ultra-violet  waves  are  being  also  emitted.  So 
then  it  appears  that  the  process  by  which  visible  light- 
waves are  emitted  is  only  a  continuation  of  the  process 
by  which  the  invisible  infra-red  waves  are  emitted.  What 
further  proofs  do  you  require  as  to  the  essentially  kindred 
nature  of  the  visible  and  invisible  waves  ?  I  have  shown 
you  that  these  infra-red  waves  behave  as  visible  light- 

1  Captain  Abney  has  shown,  however,  that  owing  to  the  want  of 
sensitiveness  of  the  eye  for  red  light,  and  its  greater  sensitiveness  for 
green  light,  the  tint  of  minimum  visible  luminosity  of  any  hot  body 
or  indeed  of  any  feebly  illuminated  body  in  a  perfectly  dark  room  is 
greenish.     This  is  true  even  of  a  light  seen  through  ruby  glass  if  the 
eye  has  been  kept  some  time  in  darkness. 

2  The  whitest  known  artificial  light  is  that  of  the  arc-lamp ;  it  is 
the  light  of  carbon  incandescent  at  about   3500°  C.      This  is  a 
temperature  considerably  lower  than  that  of  the  sun's  surface,  which 
emits  a  light  having  a  relatively  higher  proportion  of  blue  and  violet 
and  of  ultra-violet  waves.     In  fact,  when  seen  in  full  sunlight  the 
light  of  the  arc-lamp  is  decidedly  dull  and  reddish.     No  accurate 
definition  of  any  standard  of  whiteness  has  ever  been  given. 


212  LIGHT  LECT. 

waves  do  in  a  number  of  respects.  You  have  seen  that 
we  can  refract  them  with  a  lens,  disperse  them  with  a 
prism,  reflect  them  with  a  mirror,  and  absorb  them  with 
a  black  surface.  Further,  they  travel  at  the  same  rate 
across  space  as  the  visible  waves  do.  This  we  know 
from  that  which  happens  at  the  time  of  a  total  solar 
eclipse.  At  the  moment  when  the  sun's  light  ceases  to 
be  visible,  his  heat  ceases  also  to  reach  us.  When  the 
light  reappears  the  heat-waves  are  also  restored.  This 
one  fact  proves  these  heat-waves  to  be  simply  light  of 
an  invisible  kind.  But  if  you  are  not  satisfied  I  will  give 
you  yet  one  further  proof.  You  shall  see  that  they  can 
be  polarised. 

Here,  as  in  my  third  lecture  (Fig.  93,  p.  132),  stand  a 
pair  of  Nicol  prisms,  one  to  serve  as  polariser  and  the 
other  as  analyser.  The  lantern  sends  its  beams  through 
them.  Receiving  the  visible  light  on  a  paper  screen  we 
note  that  when  the  analyser  is  set  with  its  principal 
plane  parallel  to  that  of  the  polariser  light  is  transmitted  : 
but  on  rotating  the  analyser  through  a  right  angle  all 
light  is  cut  off.  That  is  a  purely  optical  experiment. 
Now  let  me  take  my  thermopile — which  I  described  to 
you  as  a  sort  of  eye  which  is  sensitive  to  the  invisible 
heat-waves — and  put  it  in  the  place  where  the  paper 
screen  was.  At  present  the  polariser  and  analyser  are 
crossed,  giving  the  "dark  field"  (p.  119).  No  light  falls 
on  the  thermopile,  nor  any  heat-waves,  for,  see,  the  spot 
of  light  from  the  galvanometer  that  indicates  the  state  of 
the  thermopile  is  at  its  zero  point.  Now  I  turn  back 
the  analyser  and  restore  the  bright  field.  At  once  the 
spot  of  light  on  the  scale  swings  over  to  the  right,  telling 


v  THE  INVISIBLE  SPECTRUM  213 

us  that  the  polarised  heat,  as  well  as  the  polarised  light, 
is  coming  through  the  analyser.  Turn  the  analyser 
back  again,  the  visible  waves  are  cut  off,  and  so  are  the 
invisible  ones,  for  the  spot  of  light  has  returned.  Now 
let  me  clinch  the  proof  by  working  entirely  with  invisible 
waves  to  the  exclusion  of  visible  ones.  Here  is  a  sheet 
of  opaque  hard  black  indiarubber,  of  ebonite  in  fact.  No 
visible  light  will  come  through  it.  But  yet,  you  observe, 
when  I  have  thus  filtered  out  the  invisible  waves,  and 
stopped  off  the  visible  ones,  still  there  come  through  the 
polariser  some  waves  which  can  warm  the  thermopile, 
and  which  can  be  cut  off  by  turning  the  analyser  to  the 
position  at  right  angles. 

This  material  ebonite  is  a  most  interesting  one  from 
the  circumstance  that  it  can  thus  act  as  a  wave -filter 
transmitting  only  the  longer  waves.  Wave-filters  (or 
ray -filters)  were  extensively  used  by  Professor  Tyndall 
in  his  lectures  on  radiant  heat :  but  I  do  not  think  he 
was  acquainted  with  the  properties  of  ebonite.  Here  is 
one  of  the  Crookes  radiometers  (Fig.  no,  p.  199).  I 
place  it  in  front  of  the  lantern  but  screen  it  at  first  by  a 
thick  sheet  of  metal  so  that  it  is  all  but  at  rest.  The 
diffused  light  in  the  room  suffices  to  make  it  turn  slowly. 
I  substitute  for  the  metal  sheet  a  sheet  of  ebonite  which 
is  equally  opaque  to  ordinary  light.  Yet  the  little  vanes 
now  run  merrily  round. 

TyndalPs  filter  for  heat-waves  consisted  of  a  cell  con- 
taining a  dark  solution  of  iodine  in  bisulphide  of  carbon. 
Here  is  one  of  the  cells,  kept  cool  by  an  outer  jacket  in 
which  cold  water  circulates.  Behind  the  wall  at  the  back 
of  the  theatre  is  a  powerful  electric  arc-lamp,  the  beams 


214  LIGHT  LECT. 

of  which  pass  into  the  theatre  through  an  aperture. 
This  beam  I  propose  to  concentrate  by  a  rock-salt 
lens,  bringing  it  to  a  focus,  after  it  has  passed  through 
the  cell  that  niters  out  the  invisible  waves  and  stops  the 
visible  ones.  First  we  make  the  experiment  without  the 
cell.  All  the  waves  visible  and  invisible  come  to  a  focus. 
Holding  at  the  focus  a  bit  of  black  paper  it  smoulders 
and  then  takes  fire.  Now  interpose  the  cell.  The 
visible  light  is  cut  off;  but  holding  the  bit  of  paper  in 
the  invisible  focus  it  again  begins  to  smoulder  and  finally 
breaks  into  visible  burning. 

And  now  I  have  to  pass  to  the  most  important  recent 
discoveries — discoveries  dating  only  from  1888 — of  some 
larger  waves  which  are  exactly  like  light -waves  in  the 
following  respects :  they  can  be  reflected,  refracted, 
absorbed,  polarised,  and  diffracted.  Yet  they  differ  in 
the  most  striking  way  from  any  of  the  waves  of  light 
that  we  have  hitherto  considered.  Their  wave-lengths, 
instead  of  being  measured  by  a  few  millionths  of  an 
inch,  may  be  several  inches,  several  yards,  or  even  several 
hundreds  of  yards  long.  I  refer  to  the  electric  waves  pre- 
dicted in  1864  by  the  late  Professor  Clerk  Maxwell,  and 
discovered  experimentally  in  1888  by  the  late  Professor 
Hertz. 

Hertz  was  occupied  with  researches  upon  electric 
sparks,  which,  under  certain  circumstances,  were  known 
to  be  oscillatory.  That  is  to  say,  each  spark  might, 
under  certain  conditions,  consist  of  a  series  of  sparks 
flying  backwards  and  forwards  along  the  same  path  with 
great  regularity  and  excessive  rapidity.  If,  for  instance, 
there  were  twenty  successive  oscillations,  each  lasting 


v  THE  INVISIBLE  SPECTRUM  215 

only  one  one  hundred-millionth  part  of  a  second,1  the 
whole  series  would  only  last  one  five-millionth  part  of  a 
second,  and  would,  of  course,  seem  to  the  eye  as  simply 
an  instantaneous  spark.  In  working  with  these  oscilla- 
tory sparks  Hertz  was  led  to  investigate  the  disturbances 
which  they  set  up  in  the  surrounding  medium,  and 


FIG.  114. 


which  are  propagated  as  waves.  To  illustrate  Hertz's 
work  I  must  have  recourse  to  a  few  diagrams.  Fig.  114 
illustrates  the  apparatus  which  is  set  up  on  the  table. 
To  produce  the  sparks  we  employ  an  induction-coil. 
The  electric  discharges  produced  by  the  coil  are  sent 
into  the  simple  apparatus  called  by  Hertzian  oscillator. 
As  you  see  it  consists  of  two  square  sheets  of  metal, 
affixed  upon  two  metal  rods  that  nearly  meet  and  are 

1  It  may  be  useful  to  note  that  since  the  velocity  of  propagation 
of  electric  waves  in  air  (or  vacuum)  is  identical  with  that  of  light 
(186,400  miles  per  second,  or  30,000,000,000  centimetres  per  second), 
the  wave-length  can  be  deduced  from  the  frequency  by  the  rule  that 
the  product  of  frequency  and  wave-length  is  equal  to  that  velocity.  In 
the  above  example,  if  the  period  is  one  one  hundred-millionth  of  a 
second,  the  frequency  is  one  hundred  million  a  second  ;  dividing 
30,000,000,000  by  100,000,000  we  get  ft?  the  •wav?-lQngtb..  3^"  centi- 
metres, or  about  ten  feet  as  the  wave-length. 


216 


LIGHT 


LECT. 


provided  with  two  well-polished  metal  balls  as  terminals. 
There  is  a  small  gap  between  which  the  sparks  are  seen 
to  pass.  But  each  such  spark  is  really  a  series  of  oscil- 
lations ;  the  electric  discharge  oscillating  backwards  and 
forwards,  not  simply  across  the  gap  Where  you  see  the 
spark,  but  from  one  end  to  the  other  of  the  apparatus. 
Suppose  the  coil  to  make  one  of  these  metal  wings  (say 
A  in  Fig.  114)  positive,  while  the  other  wing  (B  in  Fig. 
114)  is  negative.  When  the  electric  state  has  risen 
sufficiently  high,  the  air  in  the  gap  is  pierced  by  a  spark. 


OSCILLATOR. 

FIG.  115. 


RESONATOR. 


The  charge  rushes  from  A  to  B,  and  in  so  doing  over- 
charges B,  making  it  positive,  while  leaving  A  negative. 
At  once  the  charge  surges  back  again  from  B  to  A,  and 
again  back  to  B,  each  oscillation  lasting  only  about  the 
one  hundred-millionth  part  of  a  second.  The  frequency 
of  the  oscillations  depends  on  the  size  of  the  apparatus. 
At  every  oscillation  an  electric  wave  is  sent  off  from  the 
apparatus  into  the  surrounding  space,  and  is  propagated 
with  the  velocity  of  light.  The  wave  is  propagated  with 
the  greatest  intensity  in  the  directions  at  right  angles  to 
the  metal  rods  along  which  the  electricity  is  oscillating, 
and  at  right  angles  to  the  plane  of  the  metal  wings. 
Fig.  115  gives  a  front  view  of  the  oscillator,  and  also  of 
the  apparatus  called  the  resonator  used  by  Hertz  for 


v  THE  INVISIBLE  SPECTRUM  2*7 

detecting  the  waves.  The  model  on  the  table  is  made 
of  the  same  size  as  one  of  Hertz's  smaller  pieces  of 
apparatus,  the  two  metal  wings  being  each  40  centi- 
metres square,  and  the  distance  between  them  60 
centimetres.  The  wave-length  of  the  waves  emitted  is 
rather  less  than  300  centimetres,  or  nearly  10  feet. 
The  resonator  or  detector  is  a  simple  wire,  bent  into  a 
ring  so  that  its  two  ends  nearly  meet.  Hertz  demon- 
strated the  fact  that  waves  pass  from  the  oscillator  by 
holding  the  resonator  some  distance  away  from  it,  and 
observing  the  minute  electric  sparks  which  they  set  up 
in  the  small  gap  between  the  ends  of  the  wire.  But  it  is 
necessary  that  the  resonator  ring  should  be  of  the  proper 
size,  and  that  it  should  be  held  in  the  right  position. 
The  size  (in  this  example  70  centimetres  diameter)  should 
be  such  that  the  natural  period  of  oscillations  of  an  elec- 
tric current  around  the  ring  should  agree  with  the  period 
of  the  waves  emitted  by  the  oscillator.  The  position 
should  be  such  that  as  the  waves  from  the  oscillator 
reach  the  resonator  they  set  up  secondary  oscillations 
in  the  ring.  If  the  resonator  is  set  up  vertically  edge- 
ways to  the  oscillator,  no  sparks  are  produced :  the 
waves  simply  stream  past  the  resonator.  If,  however, 
the  resonator  is  held  horizontally,  and  in  the  base-line 
shown  in  Fig.  114,  sparks  may  be  detected  in  the  gap. 
Hertz  put  at  the  far  end  of  the  room  where  he  was 
working  a  great  sheet  of  metal  to  reflect  back  the  waves, 
and  then  went  about  to  different  positions  in  the  room 
exploring  the  space  to  find  at  what  points  sparks  were 
produced.  He  found  that  when  the  waves  are  thus  re- 
flected back  on  themselves  there  are  nodal  points,  just 


218 


LIGHT 


LECT. 


as  there  are  nodal  points  in  sound-waves  and  in  light- 
waves when  reflected  back.  These  nodal  points  were 
spaced  out  at  distances  apart  exactly  equal  to  half  the 
wave-length,  which  thus  could  be  precisely  measured. 

Before  Hertz's  time  it  was  indeed  known  that  there 
were  oscillating  sparks.  Fig.  116  illustrates  some  experi- 
ments which  I  myself  made1  in  the  year  1876  on  this 
subject.  I  had  an  induction  coil  connected  to  send  sparks 


I/HI 


Detector 


FIG.  116. 


across  a  small  air-gap,  A,  to  a  condenser  made  of  a  dielec- 
tric, D,  between  two  metal  plates,  P  and  Q.  I  found  that 
if  there  was  this  spark-gap  in  the  circuit  of  the  coil  I  could 
draw  secondary  sparks  at  B  from  the  outer  plate  of  the 
condenser ;  and  by  means  of  a  small  vacuum-tube  and  a 
rotating  mirror  I  proved  that  these  sparks  were  oscilla- 
tory in  character.  When  these  arrangements  were  made 
I  was  able  to  get  sparks  from  insulated  metal  Objects  in 
the  room.  These  sparks  could  be  traced  all  about  the 
room.  I  had  but  to  hold  a  knife  or  pencil-case  to  the 

1  Philosophical  Magazine ,  September  1876. 


FIG.  117. — PROFESSOR  HEINRICH  HERTZ. 


THE  INVISIBLE  SPECTRUM 


219 


door-knob  or  other  piece  of  metal  to  draw  sparks.  I 
even  did  this  :  I  took  two  door-keys  and  tied  them  on  to 
a  piece  of  wood,  so  as  almost  to  touch  one  another,  and 
with  this  detector  I  could  get  sparks  while  walking  about 
to  different  parts  of  the  room.  But  it  never  dawned 
upon  me  that  these  sparks  were  the  evidence  of  electric 
waves  crossing  the  space.  That  was  Hertz's  discovery. 
He  did  not  go  idly  about  the  room  noticing  the  sparks, 
but  explored  the  positions  where  the  sparks  were  to  be 
detected,  and  holding  his  apparatus  in  the  right  position 
to  detect  them. 

A  word  more  about  the  electric   oscillations  them- 
selves. Each  sudden  discharge  of  the  induction  coil — and 
to  make  them  sudden 
the     discharge     balls 
must  be  well  polished 
—  sets    up   a    set    of 
oscillations,    diagram- 
matically  represented 
in    Fig.    118    by    the 
upper  curve,  which  die 
away  as  time  goes  on. 
A  mechanical  analogy 
may  be  found  in  the 
vibrations  of  a  spring 
denly  release  it. 


1 A  A  A  A  A  f\F 
\J  \j  \J  v  v  v  v 


FIG.  -118. 


Bend  it  on  one  side  and  sud- 
It flies  backward  and  forward,  the 
motion  dying  out  after  a  certain  number  of  swings. 
So  trains  of  waves  are  set  up,  which  also  die  away  as 
they  travel  across  space.  But  suppose  they  fall  upon  a 
proper  resonator  or  detector,  then  they  will  set  up,  by 
their  timed  impulses,  a  sympathetic  electrical  vibration 


220 


LIGHT 


LECT. 


in  that  resonator,  the  oscillations  thus  set  up  beginning 
and  increasing  in  strength  as  wave  after  wave  arrives. 
This  is  represented  graphically  by  the  lower  curve  in 
Fig.  1 1 8.  This  corresponds,  in  fact,  to  the  way  in  which 
the  sound-waves  from  a  tuning-fork,  when  they  fall  upon 
another  tuning-fork,  will  set  it  into  sympathetic  vibration, 
provided  it  is  tuned  to  the  same  note. 

In  Fig.  119  are  represented  the  parabolic  mirrors,  each 
about  6  feet  high,  with  which  Hertz  demonstrated  the 


•  FIG.  119. 

reflexion  of  electric  waves.  At  the  focus  of  one  of  these 
mirrors  there  was  placed  vertically  an  oscillator,  an 
arrangement  to  produce  sparks  vibrating  up-and-down. 
The  waves  which  resulted  were,  of  course,  waves  of  up- 
and-down  motion — polarised  in  a  vertical  plane — which 
were  reflected  in  a  beam  straight  across  the  room  to  the 
second  mirror,  which  collected  them  and  reflected  them 
to  a  focus  upon  a  detector,  which  in  this  case  was 
straight,  not  circular,  with  a  small  spark-gap  at  its  middle, 
where  the  minute  sparks  could  be  detected. 

Many  forms  of  oscillator  or  vibrator  have  been  used 


v  THE  INVISIBLE  SPECTRUM  221 

by  different  experimenters  to  produce  electric  waves. 
Some  of  these  are  shown  in  Fig.  120.  The  first  is  one 
of  those  used  by  Hertz  himself.  Instead  of  flat  wings  of 
metal  he  used  in  this  case  cylindrical  metal  conductors. 
In  another  form,  described  as  a  dumb-bell  oscillator, 
there  were  two  large  metal  balls.  In  every  case  the 
spark-gap  was  arranged  between  two  small  highly- 
polished  metal  balls,  midway  along  the  length  of  the 
oscillator.  The  third  shape  is  one  devised  by  Professor 


^  Herts 


Hertz 


FIG.  120. 

Righi  for  making  short  waves.  Here  there  are  three 
gaps,  the  central  one  being  between  two  balls  immersed 
in  an  oil  vessel  to  prevent  premature  discharges.  The 
lowest  form  in  Fig.  120  is  also  of  Professor  Righi's  devis- 
ing. It  represents  his  apparatus  for  producing  exceed- 
ingly short  waves — less,  in  fact,  than  an  inch  long — by 
the  oscillations  set  up  between  two  spheres  to  which 
sparks  were  communicated  from  two  smaller  terminal 
balls  outside  them. 

The  next  diagram  (Fig.  121)  depicts  two  forms  of 


LIGHT 


LECT. 


oscillator  used  by  Professor  Oliver  Lodge,  of  Liverpool. 
Here  is  a  well-polished  metal  ball  supported  between 
two  smaller  balls  that  nearly  touch  it,  one  on  each  side. 
When  a  discharge  is  made  through  this  central  ball,  an 
electric  charge  surges  from  side  to  side  in  it  with  great 
vigour,  but  the  motion  dies  out  after  only  about  three  or 
four  such  surgings,  since  it  readily  radiates  its  energy 
into  space.  It  does  not  emit  a  long  train  of  waves  :  the 


FIG.  121. 


FIG.  122. 


effect  dying  out  after  about  i|-  or  2  complete  oscillations. 
The  wave-length  of  the  emitted  waves  is  about  i|-  times 
the  diameter  of  the  ball.  The  other  form,  Fig.  121^, 
shows  a  metal  cylinder  which  is  sparked  into  at  opposite 
ends  of  a  diameter  by  two  interior  balls.  This  oscillator 
emits  its  energy  less  rapidly,  and  the  oscillations  last 
longer.  It  gives  rise  to  a  train  of  waves  which  are  pro- 
pagated chiefly  straight  out  of  the  mouth  of  the  cylinder. 
Fig.  122  depicts  one  of  the  simplest  ways  of  detecting  such 


v  THE  INVISIBLE  SPECTRUM  223 

electric  waves,  and  at  the  same  time  makes  them  evident 
to  a  whole  audience.  An  ordinary  gold-leaf  electroscope 
is  provided  with  a  by-pass  of  wire  arranged  with  a  minute 
gap  (adjustable  by  a  screw)  to  break  the  continuity.  If 
the  gold  leaves  are  carefully  charged  they  will  remain 
diverging,  because  their  electric  potential  has  not  been 
raised  sufficiently  to  cause  a  discharge  across  the  gap. 
But  if  now  an  electric  wave  from  a  Hertz  oscillator — 
especially  from  an  oscillator  set  vertically  to  produce 
vertically  polarised  waves — falls  on  the  electroscope,  it 
sets  up  in  the  wire  by-pass  an  electric  surging  that  will 
overleap  the  gap  with  a  minute  spark.  And,  during  the 


FIG   123. 

time  that  the  spark  bridges  the  gap  the  gold  leaves  dis- 
charge themselves  and  fall  together. 

A  still  more  sensitive  detector  used  by  Lodge  consists 
of  a  bit  of  glass  tube  filled  loosely  with  iron  filings 
(Fig.  123)  and  joined  along  with  a  weak  voltaic  cell  in 
circuit  with  a  galvanometer.  Loose  metal  filings  or 
powders  form  a  very  bad  and  incoherent  conductor : 
hardly  any  current  passes  through  them.  But  let  an 
electric  wave  fall  on  the  tube,  instantly  the  filings  become 
— as  discovered  by  M.  Branly — an  excellent  conductor. 
So  there  results  a  movement  of  the  galvanometer,  and  of 
the  spot  of  light  reflected  by  it,  proving  that  an  electric 
wave  has  been  detected  by  the  tube. 

Fig.   124  shows  a  set  of  the  apparatus  with  which 


224 


LIGHT 


LECT. 


Professor  Lodge  repeated  and  verified  the  observations  of 
Hertz  as  to  the  optical  properties  of  these  electric  waves. 
Hertz  had  reflected  them  with  parabolic  mirrors  6  feet 
high,  and  refracted  them  with  huge  prisms  of  pitch.  He 
found  they  could  penetrate  through  wooden  floors  and 
stone  walls.  He  polarised  them  and  diffracted  them. 
Lodge  was  able  to  repeat  these  results,  but  with  an 
apparatus  of  less  heroic  dimensions.  The  sender  con- 


FIG.  124. 

sists  of  an  oscillator,  like  Fig.  121^:,  having  a  5-inch  ball 
emitting  7 -inch  waves,  enclosed  in  a  copper  box  furnished 
in  front  with  a  diaphragm  perforated  with  apertures  of 
various  sizes  to  moderate  the  radiation  to  any  desired 
degree.  As  detector,  D,  there  is  used  a  tube  full  of 
coarse  iron  filings  put  at  the  back  of  a  copper  hat,  whose 
open  end  is  turned  in  any  direction  in  which  waves  are 
to  be  received.  Wires  pass  from  the  detector  to  the 
galvanometer,  G,  and  are  enclosed  in  a  metal  tube  to 
shield  them  from  stray  radiations.  If  the  receiver  is  set 


THE  INVISIBLE  SPECTRUM 


225 


obliquely  to  the  sender  so  that  no  waves  from  the  sender 
enter  the  receiver,  the  galvanometer  will  give  no  indica- 
tions. But  if  the  waves  from  the  sender  are  reflected 
into  the  mouth  of  the  receiver  by  holding  in  front,  at  the 
proper  angle,  a  sheet  of  metal,  at  once  the  detector  is 
affected,  and  the  galvanometer  reveals  the  fact.  Simi- 
larly, as  shown  in  the  diagram,  a  prism  of  paraffin  wax 
may  be  used  to  refract  the  electric  waves  into  the  mouth 

M 


FIG.  125 

of  the  receiver.     The  picture  also  shows  a  grid  of  metal 
wires  which  can  be  used  to  polarise  the  electric  waves. 

Moje  recent  than  the  researches  of  Professor  Lodge 
are  those  of  Professor  Righi,  whose  apparatus  is  shown 
in  Fig.  125.  It  consists  of  two  parts,  a  sender  and  a 
receiver.  The  sender,  on  the  left,  consists  of  a  small 
oscillator  (that  shown  at  the  bottom  of  Fig.  120),  with 
three  spark-gaps,  the  central  gap  being  capable  of  fine 
adjustment.  In  Fig.  125  this  gap  is  between  the  ball 
marked  V,  and  one  below  it  (not  shown),  enclosed  in  a 
small  leather  capsule  filled  with  vaseline.  This  oscil- 
lator is  set  at  the  focus  of  a  parabolic  mirror,  M,  to  reflect 

Q 


226  LIGHT  LECT. 

the  waves  out  straight  to  the  right  across  the  central 
table  C.  The  detector,  D,  which  also  is  furnished  with  a 
parabolic  mirror,  is  an  optical  one.  It  is  made  of  a  film 
of  silver  upon  a  slip  of  glass,  the  film  being  divided  in 
two  across  the  middle  with  a  diamond  cut.  Across  this 
narrow  gap  minute  sparks  pass,  and  are  viewed  through 
an  eye-piece  at  D.  The  apparatus  is  quite  small  enough 
to  be  put  upon  an  ordinary  table,  and  presents  quite  the 
appearance  of  a  piece  of  optical  apparatus.  Upon  the 
central  table  can  be  mounted  reflectors,  prisms,  lenses, 
grids,  or  any  other  apparatus.  With  these  devices 
Professor  Righi  has  tracked  down  the  optical  properties 
of  the  electric  waves  varying  from  8  inches  down  to 
i  inch  in  length.  He  has  demonstrated  interference 
fringes  by  Fresnel's  mirrors,  and  with  the  biprism,  with 
thin  plates,  and  by  diffraction.  He  has  verified  the 
laws  of  refraction,  reflexion,  total  reflexion,  polarisation, 
and  of  elliptical  and  circular  oscillations.  He  also 
investigated  the  transparency  of  media  and  the  selective 
transparency  of  wood  according  to  its  grain,  a  property 
which  makes  it  polarise  the  electric  waves  just  as  tour- 
maline polarises  light.  In  short,  he  has  completed  the 
proofs  that  these  waves  possess  all  the  known  properties 
of  ordinary  light.  Other  workers  have  occupied  them- 
selves in  the  same  field.  We  are  shortly  to  hear  a 
discourse  here l  by  Professor  J.  Chunder  Bose,  of  Calcutta, 

1  Given  Friday,  January  29,  1897.  Professor  Bose's  oscillator 
is  depicted  in  Fig.  126  ;  it  is  made  of  a  small  ball  of  platinum 
between  two  smaller  balls.  Single  sparks  are  given  to  this  from  a 
small  induction-coil.  A  cylindrical  lens  of  ebonite  in  front  of  this 
oscillator  renders  parallel  the  emitted  waves.  The  complete 
apparatus  is  shown  in  Fig.  127.  The  oscillator  or  sender  S  enclosed 


THE  INVISIBLE  SPECTRUM 


227 


upon  the  polarisation  of  the  electric  wave  as  studied  by 
him,  with  an  exceedingly  elegant  apparatus  producing 
still  shorter  waves. 

But  before  I  close  I  must  show  you  at  least  some  of 

in  a  metal  tube  projects  from  a  box  A,  doubly  cased  in  metal  ; 

which  contains  the  induction-coil  and  battery.      The  detector  D 

consists  of  a  number  of 

small     metal     springs 

lightly  pressing  against 

one  another,  traversed 

by   a   current    from   a 

single    cell    C    in    the 

circuit  of  which  is  in- 

cluded  the   galvanometer   G.      The   detector  D   is   set   up   on   a 

movable  arm  on  an  optical  circle,  so  that  the  optical  properties 

of  the  electric  beam  may  be  studied.      M  is  a  plane  mirror,   N 

a  curved  mirror  for  studying  the  laws  of  reflexion,   P  is  a  prism 

of  ebonite,  T  a  special  apparatus  for  observing  the  total  reflexion 


FIG 


FIG.  127. 

at  films  of  liquid  enclosed  between  two  semi-cylinders  of  ebonite, 
H  is  a  holder  in  which  pieces  of  minerals  can  be  clamped  for 
observation.  Professor  Bose  has  found  that  many  crystalline  and 
fibrous  minerals,  such  as  epidote  and  asbestos,  polarise  the  electric 


228 


LIGHT 


LECT. 


these  effects  in  actual  experiment.  Here,  in  a  metal 
box,  is  a  small  induction  coil  actuated  by  a  couple  of 
battery  cells.  The  spark  which  this  makes  is  carried  to 
a  small  oscillator,  closely  resembling  Lodge's  (Fig.  121). 
It  is,  in  fact,  a  short  piece  of  polished  platinum  tube,  4 
millimetres  in  diameter,  between  two  small  beads  of 
platinum.  It  emits  waves  about  J  inch  long.  I  surround 
,  it  with  a  metal  bonnet  or  tube  to 
direct  the  waves  straight  out.  The 
detector  is  simply  a  tube  loosely 
filled  with  iron  filings  in  circuit  with 
a  galvanometer,  and  a  cell  made 
with  a  bit  of  iron  and  a  bit  of 
copper  dipping  into  salt  water.  It 
also  is  furnished  with  an  outer  metal 
tube  to  screen  it  from  stray  radia- 
tions. If  I  set  the  sender  and 
receiver  opposite  one  another,  the 
receiver  will  respond  whenever  a  spark  is  passed  through 
the  sender.  After  each  such  response,  I  have  to  give  a 
gentle  tap  to  the  detector  to  shake  up  the  filings  and 
send  the  galvanometer  index  back  to  zero. 

Now  I  set  the  receiver  askew  so  that  none  of  the 
waves  from  the  sender  shall  get  to  the  detector.  Spark 
after  spark  is  discharged  across  the  oscillator,  but  there 
is  no  response.  Then  I  hold  a  plate  of  metal  as  a  mirror 

beam,  since  (like  the  tourmaline  for  short  waves)  these  materials 
absorb  the  electric  vibrations  in  directions  parallel'  to  certain  axes  of 
their  structure.  Even  an  ordinary  book  possesses  for  these  waves 
a  polarising  structure  ;  the  waves  that  vibrate  parallel  to  the  leaves 
being  absorbed  more  than  those  that  vibrate  in  a  direction  trans- 
verse to  them. 


v  THE  INVISIBLE  SPECTRUM  229 

to  reflect  the  waves,  or  I  interpose  at  the  proper  angle  a 
small  prism  of  paraffin  wax.  At  once  the  detector 
responds,  proving  that  the  waves  have  been  turned 
round  the  corner,  refracted  by  the  prism. 

If  then  it  can  be  proved  that  these  electric  waves, 
though  invisible,  can  be  reflected,  refracted,  polarised, 
and  absorbed,  exactly  as  the  visible  waves  of  ordinary 
light,  have  we  not  good  reason  to  regard  them  as  one 
and  the  same  phenomenon?  By  every  test,  in  every 
physical  property,  save  only  the  accident  that  our  eye  is 
not  sensitive  to  them,  they  are  nothing  else  than  waves 
of  light.  But  if  that  is  so,  are  we  not  entitled  logically 
to  draw  the  converse  inference  that  if  light,  ordinary 
light,  behaves  in  the  same  way  and  has  all  the  same  pro- 
perties on  the  small  scale  as  these  electric  waves  on  the 
larger  scale,  then  the  little  waves  of  ordinary  light  are 
also  electric  waves  ?  That,  indeed,  was  the  brilliant 
speculation,  the  daring  theory  propounded  in  1864  by 
the  late  Professor  Clerk  Maxwell.  Basing  his  ideas  upon 
the  investigations  pursued  in  this  institution  by  Faraday, 
who  himself  ventured  first  into  this  enchanted  domain  of 
electro-optics,  Maxwell  predicted  the  properties  of  electric 
waves  in  that  famous  memoir  wherein  he  set  forth  the 
doctrine  that  light  consists  of  electric  vibrations  in  space. 
And  the  brilliant  success  of  Hertz  and  those  who  have 
followed  him  in  demonstrating  by  experiment  the  optical 
properties  of  these  waves,  is  the  abundant  justification  of 
Maxwell's  prediction. 


APPENDIX  TO   LECTURE  V 

The  Electromagnetic  Theory  of  Light 

DURING  the  first  quarter  of  the  present  century  the  wave- 
theory  of  light  successfully  displaced  the  older  corpuscular 
theory.  Young,  Fresnel,  Arago,  Biot,  and  Airy  established 
the  laws  of  physical  optics  upon  an  unimpregnable  basis  of 
undulatory  theory,  leaving  Brewster  •  the  sole  surviving 
exponent  of  the  material  nature  of  light.  But  none  of  those 
who  thus  contributed  to  establish  the  wave-theory  of  light 
could  do  much  to  elucidate  the  nature  of  that  wave-motion 
itself.  If  light  consist  of  waves  they  must  be  waves  in  or 
of  something  :  that  something  being  provisionally  called  the 
ether.  But  as  to  the  nature  of  the  ether  itself,  or  as  to  the 
particular  motions  of  it  that  were  propagated  as  waves, 
scarce  anything  was  to  be  learned  save  that  the  ether  itself 
behaved  rather  like  an  incompressible  liquid  or  solid  of 
extreme  tenuity  but  great  rigidity,  and  that  the  waves  were 
of  the  kind  classed  as  transversal  (see  p.  108). 

In  1845  Faraday  discovered  the  singular  fact  that  the 
magnet  exercises  a  peculiar  action  on  light ;  the  plane  of 
polarisation  of  a  polarised  beam  being  rotated  when  the 
beam  passes  along  a  magnetic  field. 

The  existence  of  a  relation  between  light  and  magnetism 
being  thus  established,  Faraday  proceeded  to  look  for  other 
relations,  including  the  action  of  an  electrostatic  strain  on 
polarised  light,  and  the  effect  of  reflecting  polarised  light 
at  the  polished  pole  of  a  magnet,  neither  of  which,  how- 
ever, he  succeeded  in  observing. 

In   1846  he  sent  to  the  Philosophical  Magazine  some 


APP.  ELECTROMAGNETIC  THEORY  231 

"Thoughts  on  Ray  Vibrations"  in  which  he  suggested  that 
radiation  of  all  kinds,  including  light,  was  a  high  species  of 
vibration  in  the  lines  of  force.  "Suppose,"  he  says,  "two 
bodies  A  B,  distant  from  each  other  and  under  mutual 
action,  and  therefore  connected  by  lines  of  force,  and  let  us 
fix  our  attention  upon  one  resultant  of  force  having  an 
invariable  direction  as  regards  space  ;  if  one  of  the  bodies 
move  in  the  least  degree  right  or  left,  or  if  its  power  be 
shifted  for  a  moment  within  the  mass  (neither  of  these 
cases  being  difficult  to  realise  if  A  and  B  be  either  electric 
or  magnetic  bodies),  then  an  effect  equivalent  to  a  lateral 
disturbance  will  take  place  in  the  resultant  upon  which  we 
are  fixing  our  attention  ;  for  either  it  will  increase  in  force 
whilst  the  neighbouring  resultants  are  diminishing,  or  it  will 
fall  in  force  as  they  are  increasing.  .  .  .  The  propagation 
of  light,  and  therefore  probably  of  all  radiant  action,  occupies 
time;  and,  that  a  vibration  of  the  line  of  force  should 
account  for  the  phenomena  of  radiation,  it  is  necessary  that 
such  vibration  should  occupy  time  also.  .  .  .  As  to  that 
condition  of  the  lines  of  force  which  represents  the  assumed 
high  elasticity  of  the  aether,  it  cannot  in  this  respect  be 
deficient :  the  question  here  seems  rather  to  be,  whether 
the  lines  are  sluggish  enough  in  their  action  to  render  them1 
equivalent  to  the  aether  in  respect  of  the  time  known 
experimentally  to  be  occupied  in  the  transmission  of  radiant 
force." 

In  1864  Clerk  Maxwell,  in  a  paper  in  the  Philosophical 
Transactions  on  "  A  Dynamical  Theory  of  the  Electro- 
magnetic Field,"  wrote  : — "  The  conception  of  the  propaga- 
tion of  transverse  magnetic  disturbances  to  the  exclusion  of 
normal  ones  is  distinctly  set  forth  by  Professor  Faraday  in 
his  '  Thoughts  on  Ray  Vibrations.'  The  electromagnetic 
theory  of  light,  as  proposed  by  him,  is  the  same  in  substance 
as  that  which  I  have  begun  to  develop  in  this  paper,  except 
that  in  1846  there  were  no  data  to  calculate  the  velocity  of 
propagation."  Maxwell  then  sets  out  new  equations  to 
express  the  relations  between  the  electric  and  magnetic 
displacements  in  the  medium  and  the  forces  to  which  they 
give  rise.  He  not  only  accepts  the  idea  of  Faraday  that  a 


232  LIGHT  LECT.  v 

moving  electric  charge  (as  on  a  charged  body  in  motion) 
acts  magnetically  as  an  electric  current, — a  proposition  at 
that  time  unsupported  by  any  experimental  demonstration 
—  but  goes  further  and  maintains  that  there  is  also  a 
magnetic  action  produced  during  the  production  or  release 
of  an  electric  displacement  in  a  dielectric  medium  :  in 
fact,  that  displacement-currents  in  non-conductors  produce, 
while  they  last,  exactly  the  same  magnetic  action  as  the 
equivalent  conduction -current  would  produce.  He  finds 
that  if  magnetic  methods  of  measurement  are  adopted, 
the  unit  of  electricity  arrived  at  has  a  certain  value,  while 
if  purely  electrical  methods  are  used  the  unit  has  a  different 
value.  The  relation  between  these  two  units  was  found 
to  depend  on  the  "electric  elasticity"  of  the  medium, 
and  to  be  a  velocity ;  namely,  that  velocity  with  which  an 
electromagnetic  disturbance  is  propagated  in  space.  This 
velocity  had  already  been  determined  as  a  ratio  of  units  by 
Weber  and  Kohlrausch,  who  found  it  to  be  3'i9x  io10 
centims.  per  second.  The  velocity  of  apparent  propagation 
of  an  electric  disturbance  along  a  wire  had  previously  been 
roughly  determined  by  Wheatstone  at  a  somewhat  higher 
figure.  Commenting  on  Weber's  result  Maxwell  proceeds : — 
"  This  velocity  is  so  nearly  that  of  light,  that  it  seems  we  have 
strong  reason  to  conclude  that  light  itself  (including  radiant 
heat,  and  other  radiations,  if  any)  is  an  electromagnetic 
disturbance  in  the  form  of  waves  propagated  through  the 
electromagnetic  field  according  to  electromagnetic  laws.  If 
so,  the  agreement  between  the  elasticity  of  the  medium  as 
calculated  from  the  rapid  alternations  of  luminous  vibra- 
tions, and  as  found  by  the  slow  processes  of  electrical 
experiments,  shows  how  perfect  and  regular  the  elastic 
properties  of  the  medium  must  be  when  not  encumbered 
with  any  matter  denser  than  air.  If  the  same  character  of 
the  elasticity  is  retained  in  dense  transparent  bodies,  it 
appears  that  the  square  of  the  index  of  refraction  is  equal  to 
the  product  of  the  specific  dielectric  capacity  and  the 
specific  magnetic  capacity.  Conducting  media  are  shown 
to  absorb  such  radiations  rapidly,  and  therefore  to  be 
generally  opaque."  These  two  conclusions  Maxwell  himself 


APP.  ELECTROMAGNETIC  THEORY  233 

attempted  to  verify,  and  pointed  out  an  apparent  exception 
in  the  case  of  electrolytes,  which  conduct  and  yet  are 
transparent.  In  Maxwell's  theory  every  electromagnetic 
wave  must  consist  of  two  kinds  of  displacements  both  trans- 
verse to  the  direction  of  propagation,  and  at  right  angles  to 
one  another,  one  being  an  electrostatic  displacement,  the 
other  a  magnetic  displacement.  In  this  feature  Maxwell's 
theory  reconciles  the  conflicting  views  of  Fresnel  and 
MacCullagh  respecting  the  relation  of  the  displacements  to 
the  "plane  of  polarisation."  It  is  now  known  that  the 
electric  displacements  are  at  right  angles  to  that  plane  and 
agree  with  the  Fresnel  vibrations  ;  whilst  the  magnetic  dis- 
placements are  in  the  plane  of  polarisation  as  required  by 
the  theory  of  MacCullagh.  When,  in  1874,  Maxwell 
published  his  Treatise  on  Magnetism  and  Electricity,  he 
had  already  attempted  a  further  verification  of  the  theory 
by  means  of  a  new  determination  of  the  ratio  of  the  units. 
During  the  next  ten  years  British  physicists  were  busy 
following  out  the  applications  of  the  theory,  and  testing 
its  truth  in  particular  instances.  Lord  Rayleigh  showed 
that  it  led  much  more  readily  than  the  old  elastic-solid 
theory  of  light  to  the  equations  for  double  refraction,  and  to 
the  explanation  of  the  scattering  of  light  (as  in  the  blue  of 
the  sky)  by  small  particles.  FitzGerald  applied  it  to  the 
problems  of  the  reflexion  and  refraction  of  light.  J.  J. 
Thomson  undertook  a  new  determination  of  the  ratio  of 
the  units.  Ayrton  and  Perry  pursued  a  similar  investiga- 
tion by  a  new  method.  The  same  two  observers  also 
verified  the  relation  of  the  optical  and  dielectric  properties 
in  the  case  of  gases  as  required  by  the  theory,  and  ex- 
amined the  anomalies  presented  by  ice  and  ebonite. 
Poynting  and  Heaviside  independently  deduced  -from 
Maxwell's  theory  the  proposition  that  the  energy  of  an 
electric  current  travels  by  the  medium  and  not  by  the 
wire.  Hopkinson  investigated  the  relation  between  the 
refractive  index  of  a  number  of  substances  and  their 
dielectric  inductivity ;  and  found  some  notable  deviations 
from  the  values  required  by  theory.  Lodge  added  a  number 
of  important  considerations,  and  produced  mechanical 


234  LIGHT  LECT.  V 

models  in  illustration  of  Maxwell's  ideas.  The  present 
author  investigated  the  opacity  of  tourmaline  in  relation  to 
its  conductivity ;  and  found  also,  in  accordance  with 
Maxwell's  views,  that  the  conductivity  of  the  double  iodide 
of  mercury  and  copper  increases  when  it  is  raised  to  the 
temperature  at  which  its  opacity  to  light  is  suddenly 
augmented.  Even  more  important,  because  independent 
of  Maxwell's  theory,  Dr.  Kerr  in  1876  and  1877  discovered 
by  direct  experiment  new  relations  between  light  and 
magnetism  and  between  light  and  electrostatic  strain, 
effects  which  Faraday  had  suspected,  but  sought  in  vain  to 
discover.  Lastly,  FitzGerald  had,  in  1879  and  1883, 
suggested  means  of  starting  electromagnetic  waves  in  the 
ether. 

By  the  year  1884  all  British  physicists,  except  perhaps 
Lord  Kelvin,  who  had  just  then  been  elaborating  an  inde- 
pendent spring-shell  theory  of  the  ether  as  an  improvement 
on  the  elastic-solid  theory,  had  accepted  Maxwell's  theory. 
Three  years  later  Lord  Kelvin  gave  his  adhesion.  On 
the  Continent  it  was,  however,  barely  recognised.  In 
France  it  was  quite  ignored  until  Mascart  and  Joubert 
gave  some  account  of  it  in  their  treatise  on  electricity. 
In  Germany  it  was  not  quite  so  entirely  neglected.  Von 
Helmholtz  appears  to  have  been  early  drawn  to  study  it, 
and  himself  evolved  a  new  theory  of  dielectric  action  on 
similar  lines.  Later  (in  1893)  he  applied  the  electro- 
magnetic theory  to  explain  anomalous  refraction  and 
dispersion  (see  Appendix  to  Lecture  III.,  p.  100,  above). 
It  was  von  Helmholtz  who  first  drew  the  attention  of  Hertz 
to  the  possibility  of  establishing  a  relation  between  electro- 
magnetic forces  and  dielectric  polarisation.  Boltzmann 
had  also  attempted  to  verify  Maxwell's  theory  with  respect 
to  the  relation  between  the  optical  and  dielectric  properties 
of  transparent  substances.  But,  for  the  rest,  Maxwell's 
theory  was  practically  ignored.  Boltzmann  himself  wrote 
in  1891  :  "The  theory  of  Maxwell  is  so  diametrically 
opposed  to  the  ideas  which  have  become  customary  to  us 
that  we  must  first  cast  behind  us  all  our  previous  views  of 
the  nature  and  operation  of  electric  forces  before  we  can 


APP.  ELECTROMAGNETIC  THEORY  235 

enter  into  its  portals."  Wiedemann  appears  to  have 
deemed  the  discrepancies  observed  by  Hopkinson  and 
Boltzmann  as  sufficient  to  call  in  question  the  validity  of 
the  theory,  of  which  little  notice  is  taken  in  the  volumes  of 
the  1885  edition  of  Die  Lehre  von  der  Elektricitdt.  ( Fleming 
has  in  1897  shown  that  these  discrepancies  disappear  when 
the  substances  are  cooled  in  liquid  oxygen  to  about  —  1 80° 
C.) 

In  1886  Lodge,  investigating  the  theory  of  lightning 
conductors,  carried  out  a  long  series  of  experiments  on  the 
discharge  of  small  condensers,  leading  him  to  the  observa- 
tion of  electric  oscillations  and  of  the  travelling  of  electric 
waves  as  guided  by  wires.  Hertz  taking  up  the  problem 
put  to  him  by  von  Helmholtz,  threw  himself  into  investi- 
gating the  influence  of  non-conducting  media  on  the 
propagation  of  electric  sparks.  By  March  1888  he  had 
succeeded  not  only  in  producing  electric  oscillations  and 
electric  waves  by  the  apparatus  described  above  (Fig.  1 14, 
p.  215),  but  in  demonstrating  that  these  waves  could  be 
reflected  and  refracted  like  ordinary  light. 

The  result  of  the  publication  of  Hertz's  work  was 
immediate  and  widespread.  To  those  continental  physi- 
cists who  had  hitherto  ignored  Maxwell's  theory,  or  who 
were  unaware  of  the  proofs  accumulated  by  British 
physicists,  Hertz's  work  was  nothing  short  of  a  revelation. 
Scientific  Europe  precipitated  itself  upon  the  production  of 
electric  oscillations,  as  if  eager  to  make  up  lost  headway. 
The  revelation  was  the  more  significant,  since  for  those  who 
had  not  accepted  the  ideas  of  Faraday  and  Maxwell  as  to 
action  in  the  medium,  it  meant  the  abandonment  of  all  the 
other  electrical  theories  then  extant  which  were  based  on 
the  now  untenable  principle  of  action  at  a  distance.  None 
the  less  heartily  was  Hertz's  work  welcomed  in  England  by 
those  who  were  already  disciples  of  Maxwell.  They  saw 
in  it  the  crowning  proofs  of  a  theory  which  on  other 
grounds  they  had  already  accepted  as  true.  To  adopt 
Oliver  Lodge's  words,  by  the  end  of  1888  the  science  of 
electricity  had  definitely  annexed  to  itself  the  domain  of 
optics,  and  had  become  an  imperial  science. 


APP.  A  HERTZ- WAVE  MODEL  237 

Since  1888  much  has  been  done  to  complete  the 
experimental  verification  of  the  complete  analogy  between 
electric  waves  and  waves  of  light.  Of  these  the  more 
important  are  briefly  noticed  on  pp.  221-229  above.  Suffice 
it  here  to  say  that  there  is  no  known  physical  property 
possessed  by  waves  of  ordinary  light  that  has  not  been 
found  to  be  also  correspondingly  a  property  of  the  longer 
invisible  waves  produced  by  purely  electric  means.  By 
reason  of  their  greater  wave-length  they  will  pass  through 
many  substances  opaque  to  ordinary  light,  such  as  stone 
and  brick  walls,  and  through  fogs  and  mists. 


A  Hertz-wave  Model 

Subsequently  to  the  delivery  of  these  lectures,  and  while 
this  volume  was  preparing  for  the  press,  the  author  devised 
a  wave-motion  model  to  illustrate  mechanically  the  propa- 
gation of  a  wave  from  a  Hertz  oscillator  to  a  Hertz 
resonator.  This  apparatus,  which  is  depicted  in  Fig.  129, 
should  be  compared  with  Fig.  114,  p.  215.  In  this 
model  the  "  oscillator  "  is  a  heavy  mass  of  brass  hung  by 
cords,  and  having  a  definite  time  of  swing.  The 
"  resonator "  is  a  brass  circle  hung  at  the  other  end  of  the 
apparatus  by  a  trifilar  suspension.  They  are  adjusted  by 
lengthening  or  shortening  the  cords  so  as  to  have  identical 
periods  of  oscillation.  Between  them,  to  represent  the 
intervening  medium  and  transmit  the  energy  in  waves,  is  a 
row  of  inter-connected  pendulums  (on  a  plan  somewhat 
similar  to  one  suggested  in  1877  by  Osborne  Reynolds) 
consisting  each  of  a  lead  bullet  hung  by  a  V  thread,  the 
separate  Vs  overlapping  one  another  so  that  no  bullet  can 
swing  without  communicating  some  of  its  motion  to  its 
next  neighbour.  On  drawing  the  oscillator  aside  and 
letting  it  go  it  sets  up  a  transverse  wave  which  is  propa- 
gated along  the  row  of  balls  in  a  manner  easily  followed  by 
the  eye,  and  which,  on  reaching  the  resonator  at  the 
other  end,  sets  it  into  vibration. 


LECTURE  VI 

RONTGEN    LIGHT 

Rontgen's  Discovery — Production  of  light  in  vacuum  tubes  by 
electric  discharges — Exhaustion  of  air  from  a  tube — Geissler- 
tube  phenomena — The  mercurial  pump — Crookes's-tube  phe- 
nomena— Properties  of  Kathode  light — Crookes's  shadows — 
Deflection  of  Kathode  light  by  a  magnet — Luminescent  and 
mechanical  effects — Lenard's  researches  on  Kathode  rays  in 
air — Rontgen's  researches — The  discovery  of  X-rays  by  the  lu- 
minescent effect — Shadows  on  the  luminescent  screen — Trans- 
parency of  aluminium — Opacity  of  heavy  metals — Transpar- 
ency of  flesh  and  leather — Opacity  of  bones — Absence  of 
reflexion,  refraction,  and  polarisation — Diselectrifying  effects 
of  Rontgen  rays — Improvements  in  Rontgen  tubes — Specula- 
tions on  the  nature  of  Rontgen  light — Seeing  the  invisible. 

So  many  erroneous  accounts  have  appeared,  chiefly  in 
photographic  journals,  written  by  persons  unacquainted 
with  physical  science,  that  it  seems  worth  while  in 
beginning  a  lecture  on  the  subject  of  Rontgen's  rays  to 
state  precisely  how  Rontgen's  discovery  was  made,  in 
the  language  in  which  he  himself  has  stated  it. 

"Will  you  tell  me,"  asked  Mr.  H.  J.  W.  Dam  in 
an  interview1  with  Prof.  Rontgen  in  his  laboratory  at 
Wiirzburg,  "  the  history  of  the  discovery  ?  " 

1  McClure's  Magazine,  vol.  vi. ,  p.  413. 


LECT.  vi  RONTGEN  LIGHT  239 

"There  is  no  history,"  he  said.  "  I  had  been  for  a 
long  time  interested  in  the  problem  of  the  kathode  rays 
from  a  vacuum  tube  as  studied  by  Hertz  and  Lenard. 
I  had  followed  theirs  and  other  researches  with  great 
interest,  and  determined,  as  soon  as  I  had  the  time, 
to  make  some  researches  of  my  own.  This  time  I 
found  at  the  close  of  last  October  [1895].  I  had  been 
at  work  for  some  days  when  I  discovered  something 
new. ' ' 

"What  was  the  date  ?" 

"The  8th  of  November." 

"  And  what  was  the  discovery  ?  " 

"  I  was  working  with  a  Crookes's  tube  covered  by  a 
shield  of  black  cardboard.  A  piece  of  barium  platino- 
cyanide  paper  lay  on  the  bench  there.  I  had  been 
passing  a  current  through  the  tube,  and  I  noticed  a 
peculiar  black  line  across  the  paper." 

"What  of  that  ?" 

"The  effect  was  one  which  could  only  be  produced, 
in  ordinary  parlance,  by  the  passage  of  light.  No  light 
could  come  from  the  tube  because  the  shield  which 
covered  it  was  impervious  to  any  light  known,  even  that 
of  the  electric  arc." 

"  And  what  did  you  think  ?  " 

"I  did  not  think;  I  investigated.  I  assumed  that 
the  effect  must  have  come  from  the  tube,  since  its 
character  indicated  that  it  could  come  from  nowhere 
else.  I  tested  it.  In  a  few  minutes  there  was  no  doubt 
about  it.  Rays  were  coming  from  the  tube,  which  had 
a  luminescent  effect  upon  the  paper.  I  tried  it  success- 
fully at  greater  and  greater  distances,  even  at  two  metres. 


240  LIGHT  LECT. 

It  seemed  at  first  a  new  kind  of  light.  It  was  clearly 
something  new,  something  unrecorded." 

"Is  it  light  ?" 

"  No."     [It  can  neither  be  reflected  nor  refracted.] 

"  Is  it  electricity  ?" 

"  Not  in  any  known  form." 

"What  is  it?" 

"  I  do  not  know.  Having  discovered  the  existence 
of  a  new  kind  of  rays,  I  of  course  began  to  investigate 
what  they  would  do.  It  soon  appeared  from  tests  that 
the  rays  had  penetrative  power  to  a  degree  hitherto  un- 
known. They  penetrated  paper,  wood,  and  cloth  with 
ease,  and  the  thickness  of  the  substance  made  no  per- 
ceptible difference,  within  reasonable  limits.  The  rays 
passed  through  all  the  metals  tested,  with  a  facility 
varying,  roughly  speaking  [inversely],  with  the  density  of 
the  metal.  These  phenomena  I  have  discussed  carefully 
in  my  report 1  to  the  Wurzburg  Society,  and  you  will 
find  all  the  technical  results  therein  stated." 

Such  was  Rontgen' s  own  account  given  by  word  of 
mouth.  It  is  entirely  borne  out  by  the  fuller  document, 
in  which  in  quiet  and  measured  terms  Rontgen  described 
to  the  Wurzburg  Society  his  discovery  under  the  title 
"On  a  new  kind  of  Rays,"  and  which  was  the  first 
announcement  to  the  scientific  world. 

Now  you  will  note  that  in  the  whole  passage  I  have 
read  describing  the  discovery,  there  is  not  a  word  about 
photography  from  beginning  to  end.  Photography 

1  Ueber  eine  neue  Art  von  Strahlen  (Vorlaufige  Mittheilung), 
von  Dr.  Wilhelm  Konrad  Rontgen.  (Sitzungsberichte  der  Wtirz- 
burger  physik-medic.  Gesellschaft,  1895.) 


vi  RONTGEN   LIGHT  241 

played  no  part  in  the  original  observation.  No  photo- 
graphic plate  or  sensitised  paper  was  employed.  The 
discovery  was  made  by  the  use  of  the  luminescent  screen, 
the  acquaintance  of  which  you  made  (if  you  did  not  know 
of  it  before)  at  my  fourth  lecture,  when  we  were  dealing 
with  ultra-violet  light.  On  that  occasion  I  showed  you 
a  card  partly  covered  with  platino-cyanide  of  barium 
which  has  been  in  my  possession  since  1876.  When 
exposed  to  invisible  ultra-violet  light  it  shone  in  the 
dark.  No  one  who  has  ever  used  such  a  luminescent 
screen  can  blunder  into  mistaking  it  for  a  photographic 
plate.  Such  a  screen — a  piece  of  paper  covered  with  the 
luminescent  stuff  1 — was  Rontgen  using  in  his  investi- 
gations. And  as  luminescent  screens  are  not  things  to 
be  found  lying  about  by  accident,  it  is  evident  that 
its  presence  on  the  bench  in  Rontgen' s  laboratory  on 
8th  November,  1895,  when  he  was  deliberately  investi- 
gating the  phenomena  observed  by  Lenard,  was  in  no 
sense  accidental.  That  you  may  the  better  understand 
the  precise  nature  of  Rontgen's  discovery,  we  will 
repeat  the  observation  with  the  appliances  now  at  our 
disposal. 

Before  you  stands  a  Crookes's  tube,  which  I  can  at 
any  moment  stimulate  into  activity  by  passing  through 
it  an  electric  spark  from  a  suitable  induction-coil.  It 
shines  with  visible  light,  the  glass  glowing  with  a 
beautiful  greenish-gold  fluorescence.  To  stop  off  all 

1  It  is  interesting  to  note  that  Lenard's  investigations  of  1894 
were  conducted  by  the  aid  of  a  luminescent  screen  composed  of 
paper  impregnated  with  the  wax-like  chemical  called  pentadecyl- 
paratolylketone. 

R 


242  LIGHT  LECT. 

visible  light,  I  place  over  the  Crookes's  tube  this  case 
made  of  black  cardboard,  which  cuts  off  not  only  the 
visible  rays  of  every  sort',  but  also  cuts  off  the  invisible 
rays  of  the  infra-red  and  ultra-violet  sorts.  On  the  table, 
just  below  the  tube,  lies  a  sheet  of  paper  covered  with 
platino-cyanide  of  barium — in  fact,  a  luminescent  screen. 
And,  on  passing  the  electric  discharge  through  the  shielded 
Crookes's  tube  you  will  all  see  that  this  luminescent  sheet 
at  once  shines  in  the  dark ;  while  across  it — as  those 
who  are  near  may  observe — there  falls  obliquely  a  dark 
line  which  is  simply  a  shadow  of  a  small  support  that 
stands  between  the  tube  and  the  screen.  Something 
evidently  is  causing  that  sheet  of  luminescent  paper  to 
light  up.  Can  the  effect  come  from  anywhere  else  than 
from  the  tube  ?  Try  by  interposing  things,  and  see 
whether  they  cast  shadows  on  the  paper.  The  nearest 
thing  at  hand  is  a  wooden  bobbin,  on  which  wire  is 
wound.  If  I  interpose  it,  it  casts  a  shadow  on  the  paper. 
But  looking  at  the  shadow  one  notices,  curiously  enough, 
that  while  the  wire  casts  a  decided  shadow,  the  wood 
casts  scarcely  any.  I  hold  up  the  screen  that  you  may 
see  the  shadow  more  plainly.  Yes  !  there  is  something 
coming  from  that  tube  which  causes  the  screen  to  light 
up,  and  which  casts  on  the  screen  shadows  of  things 
held  between  tube  and  screen.  This  light — if  light 
it  be — comes  from  the  tube.  But  is  it  light  ?  Light,  as 
we  know  it,  cannot  pass  through  black  cardboard.  If  it 
be  light  it  is  light  of  some  wholly  new  and  more  pene- 
trative kind.  I  move  away,  still  holding  the  screen  in 
my  hand,  to  greater  distances.  Here,  two  metres  away, 
the  screen  still  shines,  though  less  brilliantly.  And, 


vi  RONTGEN   LIGHT  243 

note,  it  shines  whether  its  face  or  its  back  be  pre- 
sented toward  the  tube.  The  rays,  having  penetrated 
the  shield  of  black  cardboard  that  encloses  the  tube,  can 
also  penetrate  the  paper  screen  from  the  back,  and  make 
the  chemically-prepared  face  shine.  Let  us  follow 
Rontgen  farther  as  he  investigated  the  penetrative  power 
of  the  rays.  I  interpose  a  block  of  wood  against  which 
a  pair  of  scissors  has  been  fixed  by  nails.  You  can  see 
on  the  screen  the  shadow  of  the  scissors ;  the  light 
passes  through  the  wood,  though  not  so  brightly,  for  the 
wood  intercepts  some  of  the  rays.  Paper,  cardboard, 
and  cloth  are  easily  penetrated  by  them.  The  metals 
generally  are  more  opaque  than  any  organic  substance, 
and  they  differ  widely  amongst  one  another  in  their 
transparency.  Thin  metal  foil  of  all  kinds  is  more  or 
less  transparent ;  but  when  one  tries  thicker  pieces  they 
are  of  different  degrees  of  opacity.  Ordinary  coins  are 
opaque.  A  golden  sovereign,  a  silver  shilling,  and  a 
copper  farthing  are  all  opaque,  but  the  lighter  metals 
such  as  tin, magnesium,  and  aluminium,  notably  the  latter, 
are  fairly  transparent.  Here  is  my  purse  of  leather  with 
a  metal  frame.  I  have  but  to  hold  it  between  the  tube 
and  the  screen  to  see  its  contents — two  coins  and  a 
ring — for  leather  is  transparent  to  these  rays.  A  sheet 
of  aluminium  about  the  twentieth  of  an  inch  thick, 
though  opaque  to  every  other  previously-known  kind  of 
light  is  for  this  kind  of  light  practically  transparent.  On 
the  other  hand  lead  is  very  opaque.  Rontgen  found 
opacity  to  go  approximately  in  proportion  to  density. 
It  is  now  found  that  those  metals  which  are  of  the 
greatest  atomic  weight  are  the  most  opaque  to  Rontgen 


244  LIGHT  LECT. 

light.  Uranium,  the  atomic  weight  of  which  is  240,  is 
the  most  opaque ;  whilst  lithium,  whose  atomic  weight  is 
only  7,  and  which  will  readily  float  on  water,  is  exceed- 
ingly transparent.  In  fact  I  have  never  yet  got  a  good 
shadow  from  lithium.  This  relation  extends  not  only  to 
the  metals  themselves  but  to  their  compounds.  Thus 
the  chloride  of  lithium  is  more  transparent  than  the 
chloride  of  zinc  or  than  the  chloride  of  silver.  Finding 
that  the  denser  constituents  were  the  more  opaque,  and 
that  while  glass  and  stone  are  tolerably  opaque  such 
substances  as  gelatine  and  leather  were  comparatively 
transparent,  it  occurred  to  Rontgen  that  bone  would  be 
more  opaque  than  flesh — and  so  it  proved :  for  inter- 
posing the  hand  between  the  tube  and  the  screen  we 
find  that  while  the  flesh  casts  a  faint  shadow  the  bones 
cast  a  much  darker  one,  and  so  we  are  able  to  see  upon 
the  luminescent  screen,  in  the  darkness,  the  shadow  of 
the  bones  of  the  hand,  and  of  the  arm.  This  is  truly 
seeing  the  invisible. 

But  now  the  investigation  took  another  turn.  So  far 
there  has  been  no  mention  of  photography.  But  the 
peculiar  penetrative  light  having  been  discovered,  and 
the  shadows  having  been  seen  on  the  luminescent  screen, 
it  was  a  pretty  obvious  step  to  register  these  shadows 
photographically.  For,  as  was  already  well  known  in 
the  case  of  ultra-violet  light,  the  rays  that  stimulate 
fluorescence  and  phosphorescence  are  just  those  rays 
which  are  most  active  chemically  and  photographically. 
Hence  it  was  to  be  expected  that  these  new  rays  would 
affect  a  photographic  plate.  This  Rontgen  proceeded 
to  verify.  He  obtained  a  photograph  of  a  set  of  metal 


vi  RONTGEN   LIGHT  245 

weights  that  were  shut  up  in  a  wooden  box.  Also  of  a 
compass,  showing  the  needle  and  dial  through  the  thin 
brass  cover.  He  then  put  his  tube  under  a  wooden- 
topped  table ;  and  laying  his  hand  on  the  table  above 
it,  and  poising  over  it  a  photographic  dry-plate,  face 
downwards,  he  threw  upon  the  plate,  by  light  which 
passed  upwards  through  the  table  top,  a  shadow  of  his 
hand.  So  for  the  first  time  he  succeeded  in  photograph- 
ing the  bones  of  a  living  hand.  It  was  the  photography 
of  the  invisible.  But,  note,  even  here  there  is  no  "  new 
photography."  The  only  photography  in  the  matter  is 
the  well-known  old  photography  of  the  dry-plate,  which 
must  first  be  exposed  and  afterwards  developed  in  the 
dark-room. 

And  now,  though  it  anticipates  somewhat  the  course 
of  this  lecture,  since  the  process  of  photographic  develop- 
ment in  the  dark-room  requires  a  little  time,  I  will  pro- 
ceed to  take  a  few  photographs  which  will  then  be  taken 
to  the  dark-room  to  be  developed,  and  will  afterwards 
be  brought  back  and  shown  you  upon  the  screen  by 
means  of  the  lantern. 

[In  the  experiments  which  followed  photographs  were 
taken  of  the  hands  of  a  boy  and  of  a  girl,  also  shadows 
cast  by  sundry  gems,  including  a  fine  Burmese  ruby,  a 
sham  ruby,  a  Cape  diamond,  and  an  Indian  diamond.] 

Retracing  our  steps  in  the  order  of  discovery  I  must 
at  once  take  you  back,  nearly  two  hundred  years,  to 
the  time  of  Francis  Hauksbee,  when,  with  the  newly 
invented  electric  machine,  and  the  newly  perfected  air- 
pump,  the  first  experiments  were  made  on  the  peculiar 
light  produced  by  passing  an  electric  spark  into  a  partial 


246  LIGHT  LECT. 

vacuum.  About  that  time  Europe  was  nearly  as  much 
excited — considering  the  state  of  knowledge  and  the 
slow  means  of  communication — over  the  "  mercurial 
phosphorus,"  as  it  was  last  year  over  the  Rontgen  rays. 
This  "  mercurial  phosphorus"  was  simply  a  little  glass 
tube,  such  as  that  (Fig.  130)  which  I  hold  in  my  hand. 
It  contains  a  few  drops  of  quicksilver;  and  the  air  that 
otherwise  would  fill  the  tube  has  been  mostly  pumped 
out  by  an  air-pump,  leaving  a  partial  vacuum.  I  have 
but  to  shake  the  tube  and  it  flashes  brightly  with  a 
greenish  light.  The  friction  of  the  mercury  against  the 


FIG.  130. 


glass  walls  sets  up  electric  discharges,  which  flash  through 
the  residual  air,  illuminating  it  at  every  motion. 

While  I  have  been  talking  to  you  an  air-pump  in  the 
basement,  driven  by  a  gas-engine,  has  been  at  work 
exhausting  a  large  oval-shaped  glass  tube.  Only  per- 
haps ^J-g-  part  of  the  air  originally  in  it  remains.  On 
sending  through  it  from  top  to  bottom  the  sparks 
from  an  induction  coil,  it  is  filled  with  a  lovely  pale 
crimson  glow,  which  changes  at  the  lower  end  to  a 
violet-coloured  tint.  On  reversing  the  connections  so 
as  to  send  the  discharge  upwards  the  violet -coloured 
part  is  seen  at  the  top.  It  shows  you,  in  fact,  the  end 


vi  RONTGEN  LIGHT  247 

at  which  the  electric  discharge  is  leaving  the  tube.  The 
pale  glow  of  this  primitive  vacuum  tube  is  rich  in  light 
of  the  ultra-violet  kind,  which,  as  you  know,  readily 
excites  fluorescence.  I  have  but  to  hold  near  it  my 
platino-cyanide  screen  for  you  to  observe  the  rich  green 
fluorescence.  My  hand  will  cast  a  shadow  on  the  screen 
if  I  interpose  it,  but  there  are  no  bones  to  be  seen  in 
the  shadow.  For  here  there  is  none  of  the  penetrative 
Rontgen  light :  the  fluorescence  is  due  to  ordinary  ultra- 
violet waves,  to  which  flesh  and  cardboard  are  quite 
opaque.  If  the  tap  is  turned  on  to  readmit  the  air  you 
see  how  the  rosy  glow  contracts  first  into  a  narrowing 
band,  then  into  a  mere  line,  which  finally  changes  into 
a  flickering  forked  spark  of  miniature  lightning ;  and  all 
is  over  until  and  unless  we  pump  out  the  air  again. 
Another  beautiful  effect  is  shown  by  use  of  an  exhausted 
glass  jar,  within  which  is  placed  a  cup  of  uranium  glass, 
as  described  fifty  years  ago  by  Gassiot.  The  discharge 
overflows  the  cup  in  lovely  streams  of  violet  colour, 
while  the  cup  itself  glows  with  vivid  green  fluorescence. 
Some  thirty  years  ago  vacuum  tubes  became  an  article 
of  commerce,  and  were  made  in  many  complex  and 
beautiful  shapes  by  the  skill  of  Dr.  Geissler  of  Bonn, 
who  devised  a  form  of  mercurial  air-pump  1  for  the  pur- 
pose of  extracting  the  air  more  perfectly;  though  the 
degree  of  vacuum,  which  sufficed  to  display  the  most 
brilliant  colours  when  stimulated  by  an  electric  discharge, 
is  far  short  of  that  which  is  requisite  in  the  modern 

1  See  the  author's  monograph  on  The  Development  of  the  Mer- 
curial Air-Pump,  published  in  1888,  by  Messrs.  E.  and  F.  N. 
Spon. 


248  LIGHT  LECT. 

vacuum  tubes  of  which  I  must  presently  speak.  Here 
is  a  Geissler's  tube  showing  wondrous  effects  when  the 
spark  discharge  is  passed  into  it.  Strange  flickering 
striations  palpitate  along  the  windings  of  the  glass  tubes 
which  themselves  glow  with  characteristic  fluorescence. 
Soda-glass  fluoresces  with  the  golden-green  tint,  lead 
glass  with  a  fine  blue,  and  uranium  glass  with  a  brilliant 
green.  The  vjolet  glow  which  appears  in  the  bulb  at 
one  end  of  the  tube  surrounds  the  metal  terminal  by 
which  the  current  leaves  the  tube,  and  is  itself  due  to 
nitrogen  in  the  residual  air.  Each  kind  of  gas  gives  its 
own  characteristic  tint.  And  with  any  kind  of  gas  within 


FIG.  131. 

the  tube  the  luminous  phenomena  are  different  at  different 
degrees  of  exhaustion. 

I  have  here  a  set  of  eight  tubes,  all  of  the  same  simple 
shape  (Fig.  131),  but  they  differ  in  respect  of  the  degree 
of  vacuum  within  them.  Platinum  wires  have  been  sealed 
through  the  ends  of  each,  one  wire  a  to  serve  as  the  anoae. 
or  place  where  the  electric  current  enters,  another  wire  k 
to  serve  as  kathode  or  place  where  the  current  makes  its 
exit  from  the  tube.  Both  anode  and  kathode  are  tipped 
with  aluminium,  as  this  metal  does  not  volatilise  so 
readily  under  the  electric  discharge.  The  small  side- 
tube  s  by  which  the  tube  was  attached  to  the  pump 
during  exhaustion  is  hermetically  sealed  to  prevent  air 
from  re-entering.  The  first  tube  of  the  set  is  full  of  air 
at  ordinary  pressure,  and  does  not  light  up  at  all.  The 


vi  RONTGEN  LIGHT  249 

length  between  anode  and  kathode  (about  12  inches)  is 
so  great  that  no  spark  will  jump  between  them.  In  the 
second  tube  the  air  has  been  so  far  pumped  away  that 
only  about  -J-  of  the  original  air  remains.  Across  this 
imperfect  vacuum  forked  brush-like  bluish  sparks  dart. 
The  third  tube  has  been  exhausted  to  about  -fa  part ; 
that  is  to  say,  J-jj-  of  the  air  have  been  removed.  It 
shows,  instead  of  the  darting  sparks,  a  single  thin  red 
line,  which  is  flexible  like  a  luminous  thread.  In  the 
fourth  tube  the  residual  air  is  reduced  to  ^  or  -g1^ 
part;  and  you  note  that  the  red  line  has  widened  out 
into  a  luminous  band  from  pole  to  pole,  while  a  violet 
mantle  makes  its  appearance  at  each  end,  though  brighter 
at  the  kathode.  In  the  fifth  tube,  where  the  exhaustion 
has  been  carried  to  about  -g-J-Q,  the  luminous  column, 
which  fills  the  tube  from  side  to  side,  has  broken  up 
into  a  number  of  transverse  striations  which  flicker  and 
dance ;  the  violet  mantle  around  the  kathode  has  grown 
larger  and  more  distinct.  It  has  separated  itself  by  a 
dark  space  from  the  flickering  red  column,  and  is  itself 
separated  from  the  metal  kathode  by  a  narrow  dark 
space.  The  degree  of  exhaustion  has  been  carried  in 
the  sixth  tube  to  about  T -Q^-Q-Q  :  and  now  the  flickering 
striations  have  changed  both  shape  and  colour.  They 
are  fewer,  and  whiter.  The  light  at  the  anode  has 
dwindled  to  a  mere  star ;  whilst  the  violet  glow  around 
the  kathode  has  expanded,  and  now  fills  the  whole  of 
that  end  of  the  tube.  The  dark  space  between  it  and 
the  metal  kathode  has  grown  wider,  and  now  the  kathode 
itself  exhibits  an  inner  mantle  of  a  foxy  colour,  making 
it  seem  to  be  dull  and  hot.  The  glass,  also,  of  the  tube 


250  LIGHT  LECT. 

shows  a  tendency  to  emit  a  green  fluorescent  light  at  the 
kathode  end.  In  the  seventh  tube  the  exhaustion  has 
been  pushed  still  farther,  only  about  -^Q^-Q-Q  part  of  the 
original  air  being  left.  The  luminous  column  has  sub- 
sided into  a  few  greyish-white  nebulous  patches.  The 
dark  space  around  the  kathode  has  much  expanded, 
and  the  glass  of  the  tube  exhibits  a  yellow-green  fluor- 
escence. In  the  eighth  tube  only  one  or  two  millionths 
of  the  original  air  are  present ;  and  it  is  now  found  much 
more  difficult  to  pass  a  spark  through  the  tube.  All  the 
internal  flickering  clouds  and  striations  in  the  residual 
gas  have  disappeared.  The  tube  looks  as  if  it  were  quite 
empty :  but  the  glass  walls  shine  brightly  with  the  fine 
golden-green  fluorescence,  particularly  all  around  the 
kathode.  If  we  had  pushed  the  exhaustion  still  farther, 
the  internal  resistance  would  have  increased  so  much 
that  the  spark  from  the  induction  coil  would  have  been 
unable  to  penetrate  across  the  space  from  anode  to 
kathode. 

To  attain  such  high  degrees  of  exhaustion  as  those  of 
the  latter  few  tubes  recourse  must  be  had  to  mercurial 
air-pumps ;  no  mechanical  pump  being  adequate  to  pro- 
duce sufficiently  perfect  vacua.  The  Sprengel  pump, 
invented  in  1865  by  Dr.  Hermann  Sprengel,  is  an  admir- 
able instrument  for  the  purpose.  But  it  was  modified 
and  greatly  improved1  about  1874  by  Mr.  Crookes, 

1  These  improvements  comprised  the  following  : — A  method  of 
lowering  the  supply-vessel  to  refill  it  with  the  mercury  that  had  run 
through  the  pump  ;  the  use  of  taps  made  wholly  of  platinum  to 
ensure  tightness  ;  the  use  of  a  spark-gauge  to  test  the  perfection 
of  the  vacuum  by  observing  the  nature  of  an  electric  spark  in  it  ; 
the  use  of  an  air-trap  in  the  tube  leading  up  to  the  pump-head  ;  the 


RONTGEN   LIGHT 


251 


AlA 

trap 


whose  form  of  pump  is  shown  in  Fig.  132.  Mercury  is 
placed  in  a  supply- 
vessel,  which  can  be 
raised  to  drive  the 
mercury  through  the 
pump,  and  lowered, 
when  empty,  to  be 
refilled.  This  vessel 
is  connected  by  a 
flexible  indiarubber 
tube  to  the  pump, 
which  consists  of 
glass-tubes  fused  to- 
gether. From  the 
pump-head  the  mer- 
cury falls  in  drops 
down  a  narrower 
tube,  called  the  fall- 
tube,  and  each  drop 
as  it  falls  acts  as  a 
little  piston  to  push 
the  air  in  front  of  it, 
and  so  gradually  to 
empty  the  space  in 
the  farther  part  of  the  tube.  A  drying-tube,  filled  with 

method  of  connecting  the  pump  with  the  object  to  be  exhausted, 
by  means  of  a  thin,  flexible,  spiral  glass  tube  ;  the  method  of 
cleansing  the  fall-tube  by  letting  in  a  little  strong  sulphuric  acid 
through  a  stoppered  valve  in  the  head  of  the  pump.  In  carrying 
out  these  developments  Mr.  Crookes  was  assisted  by  the  late  Mr. 
C.  Gimingham,  whose  later  contributions  to  the  subject  are  de- 
scribed in  the  author's  monograph  on  the  Mercurial  Air-pump. 


FIG.  132. 


252  LIGHT  LECT. 

phosphoric  acid  to  absorb  moisture,  is  interposed  between 
the  pump  and  the  vacuum-tube  that  is  to  be  exhausted. 
It  is  usual  to  add  a  barometric  gauge  to  show  the  degree 
of  vacuum  that  has  been  reached. 

Before  you,  fixed  against  the  wall,  is  a  mercury-pump 
substantially  like  Fig.  132,  but  having  three  fall -tubes 
instead  of  one,  so  as  to  work  more  rapidly.  Through 
these  fall-tubes  mercury  is  dropping  freely ;  the  pump 
being  at  the  present  moment  employed  in  the  exhaustion 
of  a  Crookes's  tube,  which  has  been  sealed  to  it  by  a 
narrow  glass  tube.  When  the  exhaustion  has  been 
carried  far  enough,  this  narrow  pipe  will  be  melted  with 
a  blow-pipe,  so  as  to  seal  up  the  tube  and  enable  it  to 
be  removed  from  the  pump. 

It  was  with  such  a  pump  as  this  that  Crookes  was 
working  from  1874  to  1875  in  the  memorable  researches 
on  the  repulsion  caused  by  radiation,  which  culminated 
in  the  invention  of  that  exceedingly  beautiful  apparatus 
the  radiometer,  or  light-mill,  which  we  were  using  in  my 
last  lecture.  From  that  series  of  researches  Mr.  Crookes 
was  led  on  to  another  upon  the  phenomena  of  electric 
discharge  in  high  vacua.  Professor  Hittorf  of  Miinster 
had  already  done  some  excellent  work  in  this  direction. 
He  had  noted  the  golden-green  fluorescence  around  the 
kathode  when  the  exhaustion  was  pushed  to  a  high 
degree;  and  he  had  found  that  this  golden  glow,  unlike 
the  luminous  column  which  at  a  lower  exhaustion  fills 
the  vacuous  tube,  refuses  to  go  round  a  corner.  He 
had  even  found  that  it  could  cast  shadows,  owing  to  its 
propagation  in  straight  lines. 

Starting  at  this  point  on  his  famous  research,  Crookes 


FIG.  133.— PROFESSOR  WILLIAM  CROOKES,  F.R.S. 


vi  RONTGEN   LIGHT  253 

investigated  the  properties  of  this  kathode  light,  and 
found  it  to  differ  entirely  from  any  known  kind  of  radia- 
tion. It  appeared  to  start  off  from  the  surface  of  the 
kathode  and  to  move  in  straight  lines,  penetrating  to 
a  definite  distance,  the  limit  of  which  was  marked  by 
the  termination  of  the  "dark  space,"  according  to  the 
degree  of  exhaustion,  and  causing  the  bright  fluorescence 
when  the  exhaustion  was  carried  so  far  that  the  dark 
space  expanded  to  touch  the  walls.  Acting  on  this  hint 
he  proceeded  to  construct  tubes  in  which  the  kathode, 
instead  of  being  as  previously  a  simple  wire,  was 


FIGS.  134,  135. 

shaped  as  a  flat  disk,  or  as  a  cup  (Figs.  134,  135).  From 
the  flat  disk  the  kathode  rays  streamed  backwards  in  a 
parallel  beam.  Crookes  regarded  these  kathode  streams 
as  flights  of  negatively-electrified  molecules  shot  back- 
wards from  the  metal  surface.  Doubtless  such  flying 
molecules  of  residual  gas  there  are ;  and  they  take  part 
in  the  phenomenon  of  discharge,  bombarding  against  the 
opposite  wall  of  the  tube.  There  are,  however,  strong 
reasons  for  thinking  that  the  kathode  rays  are  not 
merely  flights  of  "radiant  matter,"  but  that  the  flying 
molecules  are  accompanied  by  ether-waves  or  ether- 
motions  which  cause  the  fluorescence  on  the  walls  of  the 
tube.  Be  that  as  it  may,  Crookes  found  the  kathode 


254  LIGHT  LECT. 

rays  to  be  possessed  of  several  remarkable  properties. 
Not  only  could  they  excite  fluorescence  and  phosphor- 
escence to  a  degree  previously  unknown,  but  they 
exercised  a  mechanical  force  against  the  surfaces  on 
which  they  impinged.  They  cast  shadows  of  objects 
interposed  in  their  path ;  and  were  capable  of  being 
drawn  aside  by  the  influence  of  a  magnet,  just  as  if  they 
were  electric  currents. 

Here  are  some  Crookes's  tubes  which  display  the 
luminescent  effects.  At  the  top  of  the  first  is  a  small 
flat  disk  of  aluminium  to  serve  as  kathode.  From  it 
shoots  downward  a  kathode-beam  upon  a  few  Burmese 
rubies  fixed  below.  They  glow  with  a  crimson  tint 
more  intense  than  if  they  had  been  red-hot.  In  a 
similar  tube  is  a  beautiful  phenakite,1  looking  like  a 
large  diamond.  When  exposed  to  the  kathode  rays 
it  luminesces  with  a  lovely  pale  blue  tint.  In  the 
third  is  placed  a  common  whelk  shell,  which  has 
been  lightly  calcined.  As  the  kathode  rays  stream 
down  upon  it  it  lights  up  brilliantly.  And,  after  the 
electric  discharge  has  been  switched  off,  the  shell 
continues  for  some  minutes  to  phosphoresce  with  a 
persistent  glow. 

In  the  next  tube,  which  contains  a  sheet  of  mica 
painted  with  a  coat  of  sulphate  of  lime  so  that  you  may 
better  see  the  bright  trace  of  its  luminescence,  a  narrow 
kathode  ray  is  admitted  through  a  slit  at  the  bottom, 
and  extends  in  a  fine  bright  line  upwards.  Holding  a 

1  A  species  of  white  emerald  found  in  the  Siberian  emerald 
mines,  and  often  sold  in  Russia  as  a  Siberian  diamond.  It  is  not 
so  brilliant  as  a  diamond,  though  much  more  rarely  met  with. 


vi  RCNTGEN   LIGHT  255 

magnet  near  it,  I  draw  the  kathode  ray  on  one  side, 
illustrating  its  deflectibility. 

To  illustrate  the  mechanical  effect  of  the  kathode 
rays  I  take  a  Crookes's  tube,  having  at  its  ends  flat 
disks  of  metal  as  electrodes.  Between  them  is  a  nicely- 
balanced  paddle-wheel,  the  axle  of  which  runs  upon  a 
sort  of  little  railway.  On  sending  the  spark  from  the 
induction-coil  through  the  tube  the  little  wheel  is  driven 
round  and  runs  along  the  rails.  Its  paddles  are  driven 
as  if  a  blast  issued  from  the  disk  which  serves  as  kathode. 
On  reversing  the  current  its  motion  is  reversed. 

Here  (Fig.  136)  is  a  Crookes's  tube  of  a  pear  shape, 
having  a  piece  of  sheet- 
metal  in  the  form  of  a 
Maltese  cross  set  in  the 
path  of  the  kathode 
rays.  See  what  a  fine 
shadow  the  cross  casts 
against  the  broad  end 
of  the  tube ;  for  the 
whole  end  of  the  tube  FIG.  136. 

glows  with  the  characteristic  golden-green  luminescence, 
except  where  it  is  shielded  from  the  rays  by  the  metal 
cross. 

With  this  tube  I  am  able  to  show  you  a  most  interest- 
ing and  novel  experiment  discovered  only  a  few  days 
ago  by  Professor  Fleming.  If  you  surround  the  tube 
with  a  magnetising  coil  through  which  an  electric  current 
is  passed,  the  magnetic  field  produces  a  remarkable 
effect  on  the  shadow.  Instead  of  pulling  it  on  one  side 
(as  a  horse-shoe  magnet  would  do),  the  magnetising 


256  LIGHT  LECT. 

coil  causes  the  cross  to  rotate  on  itself,  and  at  the  same 
time  to  grow  smaller.  To  show  the  effect  more  con- 
veniently I  have  put  the  magnetising  coil  not  around 
the  tube  itself,  but  around  an  iron  core  beyond  the  end 
of  the  tube.  So  I  am  able  to  diminish  or  augment  the 
effect  by  simply  moving  the  tube  away  from  the  iron 
core,  or  by  bringing  it  nearer.  As  I  move  it  up,  the 
shadow  of  the  cross  contracts,  and  grows  smaller  but 
brighter.  It  also  twists  round  and  turns  completely  over 
top  for  bottom  as  it  vanishes  into  a  mere  point.  But 
just  as  it  vanishes  you  see  its  place  taken  by  a  second 
large  shadow,  which,  as  I  push  the  tube  still  closer  to 
the  magnetised  core,  grows  brighter  and  also  turns 
round  and  contracts  as  its  predecessor  did.  Its  arms 
are  more  curved  than  those  of  the  first  cross.  At  the 
same  moment  when  the  second  shadow-cross  appears  a 
third  shadow  makes  its  appearance  as  a  distorted 
annular  form  against  the  walls  of  the  tube  between  the 
metal  cross  and  the  kathode.  Its  position  is  such  that 
the  shadow  seems  to  have  been  cast  as  by  rays  diverging 
from  the  other  end  of  the  tube.  As  yet  we  know  not 
the  explanation  of  these  remarkable  facts. 

The  last  tube  of  this  set  that  illustrates  Crookes's 
researches  has  as  kathode  a  large  hollow  cup  of 
aluminium  at  the  bottom  (Fig.  137).  This  concave 
kathode  focuses  the  kathode  rays  by  converging  them 
to  a  point  in  space  a  little  above  the  centre  of  the  tube. 
Crookes  found  that  if  the  kathode  rays  were  in  this  way 
focused  upon  anything,  they  produced  great  heat. 
Glass  was  melted,  diamonds  charred,  platinum  foil 
heated  red-hot  and  even  fused  by  the  impact  of  the 


vi  RONTGEN   LIGHT  257 

concentrated  kathode  stream.  In  the  focusing-tube 
now  before  you — an  old  one,  made  more  than  ten  years 
ago — there  is  a  piece  of  thin  platinum  foil  hung  in  the 
tube  to  be  heated  by  the  rays.  But  it  has  become  dis- 
placed and  no  longer  hangs  in  the  focus.  Yet  by  hold- 
ing a  small  horse-shoe  magnet  outside  the  tube  to 
deflect  the  rays  a  little,  I  can  displace  the  focus  until  it 


FIG.  137. 


falls  upon  the  surface  of  the  platinum  foil,  which  you 
now  see  is  raised  to  a  bright  red  heat. 

Since  the  date,  now  nearly  twenty  years  ago,  when 
these  most  beautiful  and  astonishing  observations  were 
made  by  Crookes,  there  has  been  much  speculation  as 
to  the  nature  of  these  interior  kathode  rays ;  their  prop- 
erties were  so  extraordinarily  different  from  anything 
in  the  nature  of  ordinary  light  that  even  the  name  "  ray  " 

S 


258  LIGHT  LECT. 

as  applied  to  them  seemed  out  of  place.  Crookes's  own 
term,  "  radiant  matter,"  was  objected  to  as  necessarily 
implying  their  material  nature ;  and  yet  no  other  ex- 
planation of  them  seemed  reasonable  than  Crookes's 
own  suggestion  that  they  consisted  of  flights  of  electri- 
fied particles.  It  was  supposed  that  they  could  only 
exist  in  a  vacuum  tube  under  an  exceedingly  high  con- 
dition of  exhaustion. 

However  in   1894  Dr.  Philipp   Lenard,  acting  on  a 
hint  afforded   by  an   observation  of   Professor   Hertz ' 


FIG.  138. 


succeeded  in  bringing  out  the  kathode  rays  into  the 
air  at  ordinary  pressure.  For  this  purpose  he  fitted  up 
a  tube  with  a  small  window  of  thin  aluminium  foil 
opposite  the  kathode,  as  shown  in  Fig.  138.  The 
general  form  of  tube  was  the  same  as  that  previously 
used  by  Hertz,  namely,  cylindrical,  with  a  small  kathode 
disk  on  the  end  of  a  central  wire,  protected  by  an  inner 
glass  tube.  The  anode  was  a  cylindrical  metal  tube 
surrounding  the  kathode.  Upon  the  further  end  of  the 

1  Hertz  noticed  that  when  a  very  thin  metal  film  was  interposed 
inside  the  Crookes  tube,  the  glass  still  fluoresced  under  the  kathode 
discharge.  He  found  this  still  to  be  the  case  when  the  film  was 
replaced  by  a  piece  of  thin  aluminium  foil  which  was  quite  opaque 
to  light. 


vi  RONTGEN  LIGHT  259 

tube  was  cemented  a  brass  cap,  having  at  its  middle  a 
small  hole  covered  with  aluminium  foil  YO^O  inc^ 
thick.  Through  this  "window,"  when  the  tube  was 
highly  exhausted,  there  came  out  into  the  open  air  rays 
which,  if  not  actual  prolongations  of  the  kathode  rays, 
are  closely  identified  with  them.  They  can  be  deflected 
by  a  magnet — though  in  varying  degrees  depending  on 
the  internal  vacuum.  They  can  excite  luminescence. 
Lenard  explored  them  by  using  a  small  luminescent 
screen  of  paper  covered  with  a  chemical  called  penta- 
decylparatolylketone.  He  found  them  to  be  capable  of 
affecting  a  photographic  dry-plate ;  and  studied  both  by 
the  luminescent  screen  and  by  the  photographic  plate 
their  power  of  penetrating  materials.  He  found  that 
air  at  ordinary  pressure  was  not  very  transparent,  acting 
toward  them  as  a  turbid  medium.  He  found  them  to 
pass  through  thin  sheets  of  aluminium  and  even  of 
copper.  He  also  caused  them  to  affect  a  photographic 
plate  that  was  completely  enclosed  in  an  aluminium 
case,  and  to  discharge  an  electroscope  enclosed  in  a 
metal  box.  All  this  work  was  done  in  1894  and  1895 
and  duly  published.  Though  it  excited  no  public  notice, 
it  was  regarded  by  physicists  as  of  very  great  importance. 
As  you  were  told  at  the  beginning  in  Rontgen's  own 
account  of  the  matter,  his  research  began  with  the 
deliberate  aim  of  reinvestigating  the  problem  of  the 
emission  of  kathode  rays  from  the  vacuum  tube  as 
studied  by  Hertz  and  Lenard.  So  as  Lenard  had  done, 
he  employed  a  luminescent  screen  to  explore  the  rays, 
and  used  a  Crookes  tube  (Fig.  139)  of  a  form  closely 
resembling  Lenard's,  and  indeed  identical  with  that 


260  LIGHT  LECT. 

previously  employed  by  Hertz.  The  end  opposite  the 
kathode  was  simply  of  glass,  without  any  brass  cap  or 
aluminium  window.  Thus  prepared  he  found  what  I 
have  already  described,  those  mysterious  rays  which 
with  characteristic  modesty  he  described  as  "X-rays," 
but  which  will  always  be  best  known  as  Rontgen's  rays. 
They  are  not  kathode  rays,  though  caused  by  them. 
Kathode  rays  will  not  pass  through  glass,  and  are  de- 
flected by  a  magnet.  Rontgen  rays  will  pass  through 
glass  and  are  not  deflected  by  a  magnet.  They  seem 


FIG.  139- 


indeed  to  be  formed  by  the  destruction  of  the  kathode 
rays,  having  for  their  origin  the  spot  where  the  kathode 
rays  strike  against  any  solid  object,  best  of  all  against 
some  heavy  metal  such  as  platinum  or  uranium. 
Neither  are  they  ordinary  light  of  either  the  infra-red 
or  of  the  ultra-violet  kind,  though  they  resemble  the 
latter  in  their  chemical  activity  and  in  so  freely  exciting 
luminescence.  But  ultra-violet  light  can,  as  we  have  seen 
in  previous  lectures, be  reflected,  refracted,  and  polarised, 
while  Rontgen  light  cannot.1  Nor  are  Rontgen's  rays 

1  Reflexion  there  is,  but  not  of  a  regular  kind  ;  the  supposed 
cases  of  true  reflexion  announced  by  Lord  Blythswood  and  others 
belong  to  the  category  of  myths.  There  is  diffuse  reflexion  of 


vi  RONTGEN   LIGHT  261 

the  same  thing  as  Lenard's  rays ;  for  the  latter  are  in 
various  degrees  deflectible  by  the  magnet ;  and  air  is 
toward  them  relatively  much  more  opaque  than  it  is  for 
Rontgen's  rays.  Rontgen  seems  to  have  been  fortunate 
in  having  the  means  of  producing  the  most  perfect 
exhaustion  by  his  vacuum  pump :  for  on  the  perfection 
of  the  vacuum  more  than  on  any  other  detail  does  the 
successful  production  of  the  Rontgen  rays  depend.  The 
vacuum,  which  is  abundantly  good  enough  to  evoke 
luminescence,  or  to  show  the  shadow  of  the  cross,  or  to 
produce  the  heating  at  the  focus,  or  to  drive  the  "  mole- 
cule mill,"  does  not  suffice  to  generate  the  Rontgen  rays. 
For  this  last  purpose  the  exhaustion  must  be  carried 
to  a  higher  point — to  a  point  so  high  indeed  that  the 
tube  is  on  the  verge  of  becoming  non-conductive. 

Rontgen  rays  from  polished  metals,  particularly  from  zinc,  just  as 
there  is  diffuse  reflexion  of  ordinary  light  from  white  paper.  As 
to  refraction,  Perrin  in  Paris,  and  Winkelmann  in  Jena,  have  inde- 
pendently found  what  they  think  evidence  of  feeble  refraction 
through  aluminium  prisms.  But  the  deviation  (which  is  towards 
the  refracting  edge)  is  so  excessively  small  as  to  be  scarcely  dis- 
tinguishable from  mere  instrumental  errors.  Polarisation  has 
been  looked  for  by  many  skilled  observers,  using  many  materials 
including  tourmalines.  Only  one  success  has  been  alleged,  by 
MM.  Galitzine  and  Karnojitzky,  using  tourmaline  ;  but  their 
result  has  not  been  confirmed  and  is  probably  erroneous.  Neither 
has  interference  of  Rontgen  light  yet  been  shown  to  be  possible. 
Several  observers  have  professed  that  they  have  obtained  diffrac- 
tion fringes  from  which  the  wave-length  of  the  Rontgen  rays 
could  be  measured.  But  some  of  these  measurements  show  a 
wave-length  greater  than  that  of  red  light,  and  others  less  than 
that  of  ordinary  ultra-violet  :  they  are  probably  all  due  to  some 
unnoticed  source  of  error.  None  of  them  can  be  accepted  without 
subsequent  confirmation  by  other  experimenters,  and  this  is  not 
yet  forthcoming. 


262  LIGHT  LECT. 

Here  let  me  say  a  word  about  the  man  himself  and 
his  material  surroundings.  Still  in  the  prime  of  life,  at 
the  age  of  fifty-one,  Professor  Wilhelm  Konrad  Rontgen 
had  already  made  himself  a  name  among  physicists  by  his 
work  in  optics  and  electricity  before  the  date  of  the  bril- 
liant discovery  that  gave  him  wider  fame.  He  occupies 
the  chair  of  Physics  in  the  University  of  Wiirzburg  in 
Bavaria,  and  lives  and  works  in  the  physical  laboratory 
of  the  University.  The  little  town  of  Wtirzburg,  of 
61,000  inhabitants,  boasts  a  university  frequented  by 
1,490  students,  and  supported  with  an  income  of  ^41,000 
a  year,  of  which  more  than  half  is  contributed  by  the 
State.  There  are  53  professors  and  40  assistants.  Its 
buildings  comprise  a  group  of  laboratories  and  insti- 
tutes devoted  to  chemistry,  physiology,  pathology, 
mineralogy,  and  the  like.  Its  physical  laboratory,  a 
neat  detached  block  of  buildings,  wherein  also  the  pro- 
fessor has  his  residence,  is  of  modern  design.  Its 
equipment  for  the  purpose  of  research  is  infinitely 
better  than  that  of  the  University  of  London ; '  and  it  is 

1  From  a  Report  recently  presented  to  the  Convocation  of  the 
University  of  London,  it  appears  that  the  physical  and  chemical 
laboratories  of  the  University  are  practically  non-existent.  "  There 
are  three  rooms  at  Burlington  House  which  are  occasionally  used 
as  laboratories  during  examinations,  and  for  examinational  purposes 
only.  The  largest  of  these  is  a  large  hall  lit  from  the  top.  When 
used  as  a  chemical  laboratory,  it  is  fitted  up  with  working  benches 
down  the  middle  and  along  the  two  sides,  the  benches  being 
divided  into  separate  stalls  to  isolate  candidates  in  their  work.  It 
was  stated  that  the  middle  stalls  and  benches  are  taken  down  when 
the  hall  is  used  for  written  examinations,  and  are  re-erected  when 
a  chemical  examination  is  to  be  held.  In  a  second  hall,  also  lighted 
from  above,  where  frequent  written  examinations  are  held,  tem- 
porary arrangements  are  made  whenever  an  examination  in  practical 


FIG.  140.— PROFESSOR  W.  K.  RONTGEN. 


vi  RONTGEN   LIGHT  263 

expected  of  the  professor  that  he  shall  contribute  to  the 
advancement  of  science  by  original  investigations.  With 
such  material  and  intellectual  encouragements  to  research 
as  surround  the  university  professor  in  even  the  smallest 
of  universities  in  Germany,  what  wonder  that  advances 
are  made  in  science  ?  Would  that  a  like  stimulus  were 
existent  in  England.  The  Professor  of  Physics  in  the 
University  of  London  has  made  no  discovery  like  that 
of  Professor  Rontgen,  for  the  very  good  reason  that  the 
University  of  London  has  neither  appointed  any  Pro- 
fessor of  Physics,  nor  built  any  physical  laboratory  where 
he  might  work.  Neither  the  State  nor  the  municipality 
has  provided  it  with  the  necessary  funds.  Its  charter 

physics  is  to  be  held.  A  curtain  of  black  cloth  slung  across  one  end 
of  the  room  gave  partial  obscurity  over  the  tables  where  photo- 
metric and  spectroscopic  apparatus  was  placed.  The  third  room, 
sometimes  called  the  galvanometer  room,  is  a  smaller  room  in  the 
basement,  artificially  lighted,  and  used  chiefly  for  printing,  except 
at  the  times  of  examinations  in  practical  physics."  Such  is  the 
melancholy  state  of  things  in  a  University  where  everything  is  sacri- 
ficed on  the  altar  of  competitive  examinations. 

Bavaria  has  a  population  of  6,000,000.  It  supports  the  three 
Universities  of  Munich,  Erlangen,  and  Wurzburg,  with  a  total  of 
over  6,000  students,  at  a  cost  of  ^150,000  a  year,  of  which 
^93,000  is  provided  by  the  State.  London,  with  a  population  of 
5,000,000,  has  the  University  of  London,  a  mere  Examining 
Board,  to  which  come  up  for  intermediate  and  degree  examina- 
tions about  2,000  students  yearly,  of  whom  a  large  proportion  are 
from  the  provinces.  It  has  no  professors.  Its  laboratories  are  in 
the  deplorable  position  above  mentioned.  So  far  from  being 
endowed  by  the  State,  it  pays  in  to  the  State  about  ,£16,500  a 
year,  and  nominally  receives  back  about  ^"16,280  as  a  parliamen- 
tary grant.  It  receives  no  subvention  from  the  municipality.  Its 
library  is  closed  for  a  large  portion  of  the  year,  the  room  being 
used  for  examination  purposes  almost  every  day. 


264  LIGHT  LECT. 

precludes  it  from  doing  anything  for  science  except  hold 
examinations  !  Perhaps  some  day  London  may  have  a 
university  worthy  of  being  mentioned  beside  that  of 
Wiirzburg,  which  is  eleventh  only  in  size  amongst  the 
universities  of  Germany. 

Rontgen  had  so  thoroughly  explored  the  properties 
of  the  new  rays  by  the  time  when  his  discovery  was 
announced,  that  there  remained  little  for  others  to 
do  beyond  elaborating  his  work.  One  point  deserves 
notice ;  namely,  the  improvement  of  the  tubes.  Rontgen 
held  the  view  that  his  rays  originated  at  the  fluorescent 
spot  where  the  kathode  rays  struck  the  glass.  This  led 
some  persons  to  the  idea  that  fluorescence  was  advan- 
tageous. Several  workers,  however,  discovered  about 
the  same  time  that  if  the  kathode  rays  were  focused 
upon  a  piece  of  metal  the  emission  of  Rontgen  light 
became  more  copious.  When  studying  early  last  year 
the  conditions  under  which  the  rays  were  produced,  I 
found  that  the  best  radiators  are  substances  which  do 
not  fluoresce — namely,  metals.  I  found  zinc,  magnesium, 
aluminium,  copper,  and  iron  to  answer;  but  platinum 
was  better  than  these,  and  uranium  best  of  all.  Directing 
the  kathode  discharge  against  a  target  or  "  antikathode  " 
of  platinum  fixed  in  the  middle  of  the  tube,  I  carefully 
watched,  by  aid  of  a  luminescent  screen,  the  emissive 
activity  of  the  surface  during  the  process  of  exhaustion. 
After  the  stage  of  exhaustion  has  been  reached  at  which 
Crookes's  shadows  are  produced,  one  must  go  on  further 
exhausting  before  any  trace  of  Rontgen  rays  appear. 
The  first  luminosity  seems  to  come  (as  in  Fig.  141)  from 
both  front  and  back  of  the  target  at  once  j  an  oblique 


VI 


RONTGEN   LIGHT  265 


line,  corresponding  to  the  plane  of  the  "  anti kathode" 
or  target  of  metal,  being  seen  on  the  screen  between 
two  partially  luminescent  regions.  On  continuing  the 
exhaustion,  the  light  behind  dies  out  while  that  in 
front  increases,  as  in  Fig.  142,  the  rays  being  emitted 
copiously  right  up  to  the  plane  of  the  antikathode.  This 


FIG.  141.  FIG.  142. 

lateral  emission  is  quite  unlike  anything  in  the  emission 
or  reflexion  of  ordinary  light,  and  has  to  be  accounted 
for  in  any  theory  of  the  Rontgen  rays.  I  have  myself 
observed l  that  within  the  tube  there  are  some  other 
rays  given  off  in  a  similar  way,  along  with  the  Rontgen 
rays,  but  which  are  not  Rontgen  rays,  for  they  can  be 

1  See  Electrician. 


266 


LIGHT 


LECT. 


deflected  by  a  magnet,  and  more  nearly  resemble  the 
kathode  rays.  It  is  these  that  produce  on  the  glass 
wall  of  the  tube  a  well-marked  fluorescence  delimited 
(as  in  Fig.  142)  by  an  oblique  plane  corresponding  to 
the  delimitation  of  Rontgen  rays  seen  in  the  fluorescent 
screen.  The  tube  which  I  used  at  the  beginning  of 
this  lecture,  and  which  we  will  use  again  at  the  close 
of  the  lecture  to  show  you  your  own  bones,  is  of  the 
focus  type  (Fig.  143).  It  is  of  the  pattern  devised  by 
Mr.  Herbert  Jackson,  of  King's  Col- 
lege. The  concentration  of  the  kath- 
ode rays  upon  the  little  target  of 
platinum  (which  often  becomes  red-hot) 
has  the  advantage  not  only  of  allowing 
a  more  copious  emission  of  Rontgen 
rays  than  would  be  possible  if  the  anti- 
kathodal  surface  were  the  glass  wall, 
but  also  of  causing  the  Rontgen  rays 
to  issue  from  a  small  and  definite 
source  so  that  the  shadows  cast  by 
objects  are  more  sharply  defined. 
Here  are  two  still  more  recent  tubes 
(Figs.  144,  145)  constructed  for  me 
by  Mr.  Bohm,  in  which  the  focus  prin- 
ciple is  preserved ;  but  in  which  there 
is  the  improvement  that  the  anti- 
kathode  T  is  not  used  also  as  an  anode.  It  is  an 
insulated  target  of  platinum,  while  the  anodes  are 
aluminium  rings  through  which  the  cone  of  kathode 
rays  passes.  These  tubes  are  not  liable  to  blacken,  as 
is  the  case  with  tubes  in  which  the  antikathode  is  also 


FIG.  143. 


VI 


RONTGEN  LIGHT 


267 


used  as  anode.     The  tube  (Fig.  145)  has  two  concave 
electrodes,   either  or  both  of    which  may  be  used  as 


FIG.  144. 


FIG.  145. 


kathode;  it   is   a   convenient   form   for    those  cases  in 
which  an  alternating  current  is  employed. 

In  another  direction  many  efforts  have  been  made 
at  improvement  of  the  luminescent  screen.  At  first 
good  barium  plati no-cyanide  was  not  to  be  procured, 
and  hydrated  potassium  platino-cyanide  was  found  far 
superior.  But  the  good  barium  salt  now  procurable  is 


268  LIGHT  LECT. 

quite  as  luminescent,  and  is  less  troublesome  to  manage. 
One  result  of  the  ignorance  which  at  first  prevailed  as 
to  the  real  origin  of  Rdntgen's  discovery  was  that 
various  experimenters  up  and  down  the  world  supposed 
themselves  to  have  invented  something  when  they  took 
to  using  fluorescent  screens.  One  man  puts  a  fluor- 
escent screen  at  the  bottom  of  a  pasteboard  tube,  with 
a  peep-hole  lens  at  the  top,  and  calls  it  a  "  cryptoscope." 
Another,  in  another  part  of  the  globe,  puts  a  fluorescent 
screen  at  the  bottom  of  a  nice  cardboard  box  furnished 
with  a  handle  and  a  flexible  aperture  to  fit  to  the  eyes, 
and  styles  it  a  "  fluoroscope."  Both  are  useful;  but 
the  only  invention  in  the  whole  thing  is  Rontgen's. 

Within  a  fe>y  days  of  the  publication  of  Rontgen's 
discovery  another  effect,  however,  which  had  escaped 
Rontgen's  scrutiny,  was  observed  by  several  independ- 
ent observers.  It  had  been  known  for  several  years 
that  when  ultra-violet  light  falls  upon  an  electrically- 
charged  surface  it  will  cause  a  diselectrification,  but 
only  if  the  surface  is  negatively  charged.  Ultra-violet 
light  will  not  diselectrify  a  positive  charge.1  But 
Rontgen  rays  are  found  to  produce  a  diselectrification 
of  a  metal  surface  (in  air)  whether  the  charge  be  posi- 
tive or  negative.  Here  is  a  convenient  arrangement  for 
exhibiting  the  experiment.  An  electroscope  made  on 
Exner's  plan  with  three  leaves — the  central  one  a  stiff 
plate  of  metal — is  charged,  and  then  exposed  to  Ront- 

1  Ultra-violet  light  will  not  diselectrify  a  metal  surface  in  air 
unless  that  surface  is  negatively  charged.  I  have  observed  a  case, 
however,  in  which  a  positively-electrified  body  was  discharged  by 
ultra-violet  light,  but  it  was  not  a  metal  surface,  nor  in  air. 


VI 


RCNTGEN   LIGHT 


269 


FIG.  146. 


gen  light.  The  three  leaves  are  made  of  aluminium, 
aluminium  foil  being  better  than  leaf-gold  for  electro- 
scopes. They  are  supported  within  a  thin 
flask  of  Bohemian  glass  entirely  enclosed, 
except  at  the  top,  in  a  mantle  of  trans- 
parent metallic  gauze.  After  the  leaves 
have  been  charged — either  positively  by  a 
rod  of  rubbed  glass,  or  negatively  by  a  rod 
of  rubbed  celluloid — a  metal  cap  is  placed 
over  the  top  (Fig.  146). 

The  leaves,  being  thus  completely  sur- 
rounded by  metal,  are  effectually  screened 
from  all  external  electrical  influences.  My 
electroscope  is  now  charged.  To  enable  you  to  see 
the  effect  better,  a  beam  of  light  is  directed  upon  it, 
throwing  a  magnified  shadow  of  the  leaves  upon  the 
white  screen.  Then,  exposing  the  electroscope  to 
Rontgen  light  from  a  focus  tube  situated  some  18 
inches  away,  you  see  the  leaves  at  once  closing  together, 
proving  the  diselectrification.  It  succeeds  whether  the 
charge  be  positive  or  negative  in  sign. 

It  now  only  remains  for  me  to  exhibit  to  you  the 
photographs  which  were  taken  at  the  beginning  of  this 
lecture,  and  a  number  of  others  prepared  as  lantern- 
slides.  In  Figs.  147,  148  we  have  the  hand  of  a  poor 
child  aged  thirteen,  a  patient  in  St.  Bartholomew's 
Hospital.  She  was  brought  to  my  laboratory  that  the 
deformities  of  her  hands  might  be  examined.  The  first 
of  the  two  plates  was  insufficiently  exposed,  with  the 
result  that  the  bones  scarcely  show  through  the  flesh  at 
all.  The  second  plate  was  over-exposed,  and  the  rays 


270  LIGHT  LECT. 

have  penetrated  the  flesh  so  thoroughly  that  only  the 
bones  appear. 

Fig.  149  is  the  hand  of  a  child  of  eleven  years  old. 
In  a  child's  hand  the  bones  are  not  yet  completely 
ossified,  their  ends  being  still  gelatinous  and  trans- 
parent, so  that  there  seem  to  be  gaps  between  them. 
Compare  this  with  the  hand  of  a  full-grown  man,  and 
you  will  see  how  age  changes  the  aspect  of  the  bones. 

Fig.  150  is  the  hand  of  a  full-grown  woman.  You 
will  observe  in  the  case  of  the  lady's  rings  that  the 
diamonds  are  transparent,  while  the  metal  portion 
casts  a  shadow  even  through  the  bones.  These  two 
photographs  were  taken  by  Mr.  J.  W.  Gifford,  of  Chard, 
an  early  and  most  successful  worker  with  Rontgen  rays. 

Fig.  151  is  the  hand  of  Lord  Kelvin,  and  shows 
traces  of  age,  and  of  a  tendency  to  rheumatic  deposits. 

Fig.  152  is  the  hand  of  Mr.  Crookes,  and  though 
a  knottier  hand,  shows  some  points  of  resemblance 
with  that  of  Lord  Kelvin. 

Fig.  153  is  the  hand  of  Sir  Richard  Webster.  The 
shadow  is  interesting  as  showing  not  only  an  athletic 
development,  but  as  revealing,  embedded  in  the  flesh 
between  the  thumb  and  the  first  finger,  two  small 
shot,  the  result  of  a  gunshot  wound  received  many  years 
previously.  This  photograph  and  the  two  preceding 
are  from  the  series  taken  by  Mr.  Campbell-Swinton, 
who  was  first  in  England  to  put  into  practice  this  newest 
of  the  black  arts. 

By  the  courtesy  of  Mr.  Campbell-Swinton  I  am  also 
able  to  show  you  a  number  of  other  slides — the  hand 
of  Lord  Rayleigh ;  the  hand  of  a  lady  with  a  needle 


FIG.  149. — Hand  of  Child,  aged  eleven  years. 
(Photo,  by  Mr.  J.  IV.  Gifford). 


OF   THB    **F 

UNIVERSITY 

j^f  ^r~ 


FIG.  150. — Hand  of  full-grown  Woman. 
(Photo,  by  Mr.  J.  IK  Gifford). 


FIG.  151.— Hand  of  Professor  Rt.  Hon.  Lord  Kelvin. 


I 


FIG.  152.— Hand  of  Professor  W.  Crookes,  F.R.S. 


OF   THK 

UNIVERSITY 


FIG   153.— Hand  of  Rt.  Hon.  Sir  Richard  Webster,  M.P, 


FIG.  154. 


FIG. 


vi  RONGTEN   LIGHT  271 

embedded  in  the  palm ;  a  hand  terribly  swollen  with  the 
gout ;  a  foot,  showing  the  heel-bone  and  the  smaller 
bones  down  to  the  toes,  as  well  as  the  bones  in  the 
ankle ;  a  view  through  the  left  shoulder  of  a  young  lady, 
showing  her  ribs,  shoulder-blade,  and  collar-bone;  the 
torso  of  a  young  man,  showing  his  ribs,  and,  dimly,  his 
heart,  like  a  central  dark  shadow  with  a  triangular  apex 
pointing  down  toward  the  right,  that  is,  to  his  left  side ; 
lastly,  the  shadow  of  a  living  head,  showing  all  the 
vertebrae  of  the  neck. 

Here,  again,  is  the  shadow  of  a  newly-born  child, 
taken  by  Mr.  Sydney  Rowland.  Note  the  imperfect 
state  of  the  bones  in  the  hands. 

Passing  from  human  objects,  we'  will  look  at  the 
shadows  of  a  few  animals.  These  are  a  chameleon, 
giving  a  clear  view  not  only  of  its  skeleton  but  of  the 
internal  organs;  a  mouse;  a  frog;  and  some  fishes. 
The  next  slide  was  taken  from  an  Egyptian  mummy  in 
its  wrappings.  Before  this  photograph  was  taken  there 
was  some  dispute  as  to  whether  it  was  the  mummy  of  a 
cat  or  of  a  girl.  The  photograph  sets  the  question 
entirely  at  rest. 

Earlier  in  my  lecture  I  mentioned  that  glass  is  tolerably 
opaque  to  these  rays.  Of  this  you  have  a  proof  in  the 
next  photograph  (Fig.  154),  which  represents  a  pair  of 
spectacles  photographed  while  lying  in  their  case,  the 
covering  of  which,  in  shagreen,  shows  all  the  markings 
peculiar  to  the  shark's  skin,  with  which  the  case  was 
covered.  The  next  photograph  by  Mr.  Campbell- 
Swinton  enables  you  to  read  the  contents  of  a  sealed 
letter  which  he  received.  His  also  is  the  next  picture 


272 


LIGHT 


LECT. 


(Fig.  155),  which  is  the  photographed  shadow  of  an 
aluminium  cigar-case,  containing  two  cigars.  And 
lastly  (Fig.  156),  I  exhibit  to 
you  a  photograph  of  two  ruby 
rings.  By  gaslight  the  gems 
.of  one  are  not  distinguishable 
from  those  of  the  other;  and 
in  broad  daylight  it  would  take 
an  expert  to  pronounce  between 
them.  But  when  viewed  or 
photographed  by  Rontgen  light 
there  remains  no  manner  of 
doubt.  The  rubies  of  one  ring 
are  true  Burmese  rubies,  and 
they  appear  transparent.  The 
others  are  imitation  rubies 
made  of  ruby-coloured  glass,  and  appear  quite  opaque. 

You  will  have  noticed  that  I  have  spoken  of  these 
rays  as  "  Rontgen  light."  But  are  we  really  justified  in 
calling  it  light  ?  It  is  invisible  to  our  eyes  •  but  then  so 
also  is  ordinary  ultra-violet  light,  and  so  is  infra-red  light, 
and  Hertzian  light.  And  there  are  other  kinds  of  light 
too,  amongst  them  one  discovered  during  last  year  by 
M.  Becquerel  '  and  myself,  which  are  invisible.  But  if 

1  M.  Henri  Becquerel  (see  Comptes  Rendus,  cxxii.  pp.  559,  790, 
etc.)  and  I  myself  (see  Philosophical  Magazine,  July,  1896,  p.  103) 
quite  independently  discovered  some  invisible  radiations  that  are 
emitted  by  uranium  salts,  and  by  the  metal  uranium,  which  can 
affect  photographic  plates,  and  will  pass  through  a  sheet  of  alu- 
minium or  of  cardboard.  But  they  are  not  the  same  as  Ront- 
gen rays,  since,  as  M.  Becquerel  has  shown,  they  can  be  reflected, 
refracted,  and  polarised.  They  also  produce  diselectrification. 


FlG- 


vi  RONTGEN   LIGHT  273 

the  Rontgen  light  can  be  neither  reflected  nor  refracted, 
neither  diffracted  nor  polarised,  what  reason  have  we 
for  calling  it  light  at  all  ?  In  fact,  direct  proof  that  it 
consists  of  transverse  waves  is  wanting.  Many  conjec- 
tures have  been  formed  respecting  its  nature.  Rontgen 
himself  suggested  that  it  might  consist  of  longitudinal 
vibrations.  Others  have  suggested  ether  streams,  ether 
vortices,  or  even  streams  of  minute  corpuscles.  At  one 
time  the  notion  that  it  might  be  simply  an  extreme  kind 
of  ultra-violet  light  of  excessively  minute  wave-length 
was  favoured  by  physicists,  who  were  disposed  to  explain 
the  absence  of  refraction,  and  the  high  penetrative  power 
of  the  rays  upon  von  Helmholtz's  theory  of  anomalous 
dispersion,  according  to  which  the  ultra-violet  spectrum 
at  the  extreme  end  ought  to  double  back  on  itself. 

The  most  probable  suggestion  yet  made,  and  the  only 
one  that  seems  to  account  for  the  strange  lateral  emission 
of  the  rays  right  up  to  the  plane  of  the  antikathode  (see 
Fig.  142,  p.  265),  is  that  of  Sir  George  Stokes.  Stokes's 
view  is  that  while  all  ordinary  light  consists  of  trains  * 
of  waves  (Fig.  68,  p.  112),  in  which  each  ripple  is  one  of  a 
series  that  gradually  dies  away,  the  Rontgen  light  con- 
sists of  solitary  ripples,  each  of  not  more  than  one  or 

There  can  be  no  question  that  these  rays,  which  are  due  to  a  sort 
of  invisible  phosphorescence,  consist  of  transverse  vibrations  of  a 
very  high  frequency  :  that  is,  they  are  ultra-violet  light  of  a  very 
high  order. 

1  It  has  long  been  known  from  the  experiments  of  Fizeau,  that 
in  ordinary  light  each  train  consists  on  the  average  of  at  least 
50,000  successive  vibrations  ;  for  it  is  possible  to  produce  inter- 
ference of  light  between  two  parts  of  a  beam  which  have  traversed 
lengths  differing  by  more  than  50,000  wave-lengths.  Michelson 
has  gone  far  beyond  that  number.  See  the  footnote  on  p.  112. 

U 


274  LIGHT  LECT. 

one  and  a  half  waves.  According  to  Stokes  the  Rontgen 
light  is  generated  at  the  antikathode  by  impact  of  the 
flying  negatively-electrified  molecules  (or  atoms)  which 
constitute  the  kathode  stream.  At  the  moment  when 
each  of  these  flying  molecules  strikes  against  the  target 
and  rebounds,  there  will  be  a  quiver  of  its  electric 
charge ;  in  other  words,  the  charge  on  the  molecule  will 
perform  an  oscillation.  Now  that  electric  oscillation 
will  be  executed  across  the  molecule  in  a  direction  gen- 
erally normal  to  the  plane  of  the  target,  and  will  give  rise 
to  an  electro-magnetic  disturbance  which  will  be  propa- 
gated as  a  wave  in  all  directions,  except  where  stopped 
by  the  metal  of  the  target.  And  this  oscillation  being 
of  excessively  short  period,  and  dying 
out  after  about  one  or  two  (Fig.  157) 
complete  periods,  will  generate  a  wave, 
/-^^  s  which,  though  of  a  frequency  as  high  as, 
\J  or  even  higher  than,  that  of  ordinary 

ultra-violet  light,  and  therefore  capable 
FIG.  157-  Of  producing  kindred  effects,  will  not  be 

capable  of  being  made  to  interfere,  nor  to  undergo 
regular  refraction  or  reflexion,  because  it  does  not 
consist  of  a  complete  train  of  waves.  Here  is  a  model 
intended  roughly  to  illustrate  the  theory.  An  iron  hoop 
(Fig.  158)  which  can  be  thrown  or  swung  against  the 
wall  represents  the  flying  molecule.  The  electric  charge 
which  it  carries  is  typified  in  the  model  by  a  lump  of 
lead  capable  of  sliding  on  a  transverse  wire,  and  held 
centrally  by  a  pair  of  spiral  springs.  When  this  model 
molecule  is  caused  to  strike  against  the  wall  and  rebound, 
the  leaden  mass  is  disturbed,  and  executes  an  oscillation 


vi  R6NTGEN   LIGHT  275 

to  and  fro  along  the  wire.  The  oscillation  dies  out  after 
about  i^  periods.  Now,  suppose  this  oscillation  to  set 
up  a  transverse  wave  in  surrounding 
space.  Though  it  consists  of  but 
i$  ripples,  they  would  be  propagated 
outward  just  as  trains  of  waves  are. 
And  if  there  were  millions  of  such 
flying  molecules  in  operation,  these 
solitary  ripples  might  come  in  mil- 
lions one  after  the  other,  but  not* 
regularly  spaced  out  behind  one 
another  like  the  trains  of  waves 
constituting  ordinary  light  This  is 
but  a  gross  and  rough  illustration  of  Stokes' s 
thesis ;  but  it  must  suffice  for  the  present. 

But  I  cannot  close  this  course  of  lectures  without 
one  word  as  to  the  possibilities  which  this  amazing  dis- 
covery of  the  Rontgen  light  has  opened  out  to  science. 
It  is  clear  that  there  are  more  things  in  heaven  and 
earth  than  are  sometimes  admitted  to  exist.  There  are 
sounds  that  our  ears  have  never  heard :  there  is  light 
that  our  eyes  will  never  see.  And  yet  of  these  inaudible, 
invisible  things  discoveries  are  made  from  time  to  time 
by  the  patient  labours  of  the  pioneers  in  science.  You 
have  seen  how  no  scientific  discovery  ever  stands  alone  : 
it  is  based  on  those  that  went  before.  Behind  Rontgen 
stands  Lenard;  behind  Lenard,  Crookes ;  behind 
Crookes  the  line  of  explorers  from  Boyle  and  Hauksbee 
and  Otto  von  Guericke  downwards.  We  have  had 
Crookes's  tubes  in  use  since  1878,  and  therefore  for 
nearly  twenty  years  Rontgen' s  rays  have  been  in  exist- 


276  LIGHT  LECT.  vi 

ence,  though  no  one,  until  Rb'ntgen  observed  them  on 
8th  November,  1895,  even  suspected1  their  presence 
or  surmised  their  qualities.  And  just  as  these  rays 
remained  for  twenty  years  undiscovered,  so  even  now 
there  exist,  beyond  doubt,  in  the  universe,  other  rays, 
other  vibrations,  of  which  we  have  as  yet  no  cognisance. 
Yet,  as  year  after  year  rolls  by,  one  discovery  leads  to 
another.  The  seemingly  useless  or  trivial  observation 
made  by  one  worker  leads  on  to  a. useful  observation  by 
another \  and  so  science  advances,  "creeping  on  from 
point  to  point."  And  so  steadily  year  by  year  the  sum 
total  of  our  knowledge  increases,  and  our  ignorance  is 
rolled  a  little  further  and  further  back ;  and  where  now 
there  is  darkness,  there  will  be  light. 

1  It  is  but  fair  to  Professor  Eilhard  Wiedemann  to  mention  that 
in  August  1895  he  described  some  "  discharge-rays  "  (Entladungs- 
strahlen)  inside  a  vacuum  tube,  which,  though  photographically 
active,  refused  to  pass  through  fluor-spar,  and  were  incapable  of 
being  deflected  by  a  magnet.  But  their  properties  differ  from 
Rontgen  rays  in  some  other  respects. 


APPENDIX   TO   LECTURE  VI 

OTHER    KINDS    OF     INVISIBLE    LIGHT 

UPON  the  discovery  by  Rontgen  of  the  rays  that  bear  his 
name  it  was  natural  that  the  inquiry  should  be  raised  whether 
there  exist  any  other  rays  having  penetrative  properties  in 
any  degree  similar.  Lenard's  rays,  discovered  in  1894,  to 
which  some  reference  is  made  on  p.  258  above,  have-  the 
power  of  penetrating  thin  sheets  of  metal  and  of  producing 
photographic  action  as  well  as  of  discharging  electrified 
bodies.  But  they  differ  from  Rontgen's  rays  in  their  pene- 
trative power,  for  air  is  relatively  opaque  to  them.  Also 
they  are  deflected  in  varying  degrees  by  the  magnet. 
Wiedemann's  "  discharge-rays,"  briefly  metioned  above, 
are  further  described  on  p.  281. 

No  other  source  than  that  of  the  highly-exhausted 
vacuum  tube  under  electric  stimulation  has  yet  been  dis- 
covered for  Rontgen's  rays.  Many  persons  have  supposed 
Rontgen's  rays  to  be  produced  by  electric  sparks  in  the 
open  air,  simply  because  such  sparks  will  fog  photographic 
plates  and  cause  images  of  coins  and  other  metal  objects  in 
contact  with  the  plates  to  impress  images  upon  them. 
These  images  are,  however,  due  to  direct  electric  action. 
They  are  not  produced  when  a  sheet  of  aluminium  is  so 
interposed  as  to  screen  off  all  direct  electrical  action. 

In  sunlight  there  do  not  appear  to  be  any  Rontgen  rays, 
nor  yet  in  the  light  of  the  electric  arc  ;  for  neither  of  these 
sources  contains  any  rays  that  will  affect  a  photographic  plate 
that  is  protected  by  an  aluminium  sheet. 

There  are,  however,  some  kinds  of  light  that,  like  Rb'nt- 


278  LIGHT  LECT.  vi 

gen's  rays,  will  pass  through  aluminium  or  through  black 
cardboard,  and  produce  photographic  effects.  These  are 
worthy  of  some  notice. 

Becquerel's  Rays. — Early  in  1896  M.  Henri  Becquerel, 
as  mentioned  on  p.  272,  and  the  author  of  this  book  in- 
dependently, made  the  observation  that  some  invisible  radia- 
tions are  emitted  from  some  of  the  salts  of  the  metal 
uranium,  as,  for  example,  the  nitrate  of  uranyl  and  the 
fluoride  of  uranium  and  ammonium.  These  and  other  salts 
of  uranium,  whether  in  the  dark  or  in  the  light,  emit  a  sort 
of  invisible  light,  which  can  pass  through  aluminium  and 
produce  on  a  photographic  plate  shadows  of  interposed  metal 
objects.  This  effect  appears  to  be  due  to  an  invisible 
phosphorescence  of  a  persistent  sort.  Just  as  luminous 
paint  goes  on  emitting  visible  light  for  many  hours  after  it 
has  been  shone  upon,  so  these  substances  go  on  month 
after  month  emitting  an  invisible  light.  Hence  the  phe- 
nomenon is  known  as  one  of  hyper-phosphorescence.  It  is 
significant  to  note  that  ordinary  luminous  paint,  which  ceases 
after  a  few  days  to  emit  any  light  of  the  visible  kind,  will 
continue,  even  for  six  months,  to  emit  in  the  darkness  an 
invisible  radiation  of  light  that  will  fog  a  photographic 
plate. 

Some  time  after  the  original  discovery,  Becquerel  observed 
that  metallic  uranium  far  surpasses  the  salts  of  that  metal 
in  the  power  of  hyper-phosphorescence. 

The  following  is  a  summary  1  of  Becquerel's  observa- 
tions : — A  photographic  plate  was  enclosed  in  a  double 
layer  of  black  paper,  over  which  was  placed  a  thin  crust  ot 
the  transparent  crystals  of  the  double  sulphate  of  uranyl  and 
potassium.  After  several  hours  of  exposure  to  the  sun  the 
plate  was  found  to  have  been  affected.  It  was  also  affected 
when  a  sheet  of  aluminium  was  interposed,  but  a  coin 
placed  between  the  crystals  and  the  plate  cast  a  photo- 
graphic shadow.  The  experiment  also  succeeds  if  a  sheet 
of  glass  is  interposed.  This  double  sulphate,  when  examined 

1  See  paper  by  M.  Sagnac  in  Journal  de  Physique,  May,  1896, 
p.  193,  and  sundry  papers  by  M.  Becquerel  in  the  Comptes  Rendus. 


APP.         OTHER   KINDS   OF  INVISIBLE    LIGHT          279 

in  the  phosphorescope,  is  found  to  phosphoresce  for  but 
T^nth  sec.  after  exposure  to  light.  Nevertheless  it  con- 
tinues to  emit  photographically  active  rays  for  many  hours 
without  being  exposed  to  light.  The  persistence  of  these 
non-luminous  rays  is  incomparably  greater  than  that  of  the 
visible  luminescence.  The  invisible  rays  are  enfeebled  by 
passing  through  either  thin  glass  or  sheet  aluminium. 

Other  substances  were  tried.  M.  Charles  Henry  had 
found  sulphide  of  zinc,  and  M.  Niewenglowski  had  found 
sulphide  of  calcium  to  emit,  under  stimulus  of  Rontgen  rays, 
rays  that  traversed  opaque  bodies.  M.  Troost  found  hexa- 
gonal blende,  previously  exposed  to  sunlight,  to  photograph 
through  cardboard.  M.  Becquerel  found  that  the  special 
kind  of  sulphide  of  calcium  which  luminesces  blue  or  green 
gave  at  first  a  strong  photographic  action  through  aluminium 
2  mm.  thick.  Later  the  same  substances  refused  to  work. 
Zinc  sulphide  (hexagonal  blende)  similarly  failed.  Becquerel 
was  unable  to  revivify  the  sulphides  of  calcium,  either  by 
warming  or  by  cooling  to  —  20°  C.  and  exposing  to  mag- 
nesium light,  or  by  exposing  them  to  electric  sparks.  In 
the  case  of  uranyl  salts,  neither  magnesium  light  nor 
Crookes  tube  radiation  (to  which  they  are  opaque)  augment 
their  emission  of  the  invisible  rays.  Daylight  quickens 
them  a  little.  But  the  double  sulphate  of  uranyl  and 
potassium  is  quickened  by  exposure  to  the  arc-light  or  to 
that  of  sparks  from  a  Leyden  jar.  Uranous  salts,  which  are 
not  phosphorescent,  work  as  strongly  as  uranic  salts. 
Crystals  of  uranium  nitrate  melted  and  crystallised  in  the 
dark,  or  dissolved,  and  which  are  not  fluorescent  in  the 
ordinary  sense,  work  just  as  well  as  crystals  that  have  been 
exposed  to  light. 

Becquerel's  rays  possess,  like  ultra-violet  light  and  like 
Rontgen's  rays  (though  to  a  lesser  degree  by  reason  of 
their  lesser  intensity),  the  property  of  diselectrifying  charged 
bodies.  Using  an  electroscope  designed  by  M.  Hurmuzescu, 
enclosed  in  metal,  he  placed  a  lamina  of  the  crystalline 
mass  of  sulphate  of  uranyl  and  potassium  against  an 
aluminium  window  0-12  mm.  thick.  The  leaves,  which 
had  an  initial  divergence  of  18°,  collapsed  in  2  hours  55 


280  LIGHT  LECT.  vi 

minutes.  They  collapsed  in  about  one-fourth  the  time  when 
the  lamina  was  placed  inside  under  the  leaves. 

These  rays  are  absorbed  by  air.  Water  is  transparent 
to  them.  Metallic  solutions  are  transparent,  as  also  are 
wax  and  paraffin.  Uranium  glass  and  red  glass  (thickness 
2  mm.)  are  fairly  opaque.  Native  sulphur  is  transparent  ; 
calc-spar  not  very  transparent  ;  quartz  is  more  opaque  than 
calc-spar.  Tin  is  more  opaque  than  aluminium  ;  and  cobalt 
glass  more  opaque  than  either.  These  rays  go  relatively 
more  easily  through  metals  than  do  Rontgen's  rays.  Copper 
(o-i  mm.  thick)  is  very  transparent ;  platinum  nearly  as 
much.  Silver  and  zinc  allow  these  rays  to  pass,  but  lead 
0-36  mm.  thick  is  opaque.  These  measurements  were  made 
electroscopically.  Quartz  5  mm.  thick  is  less  absorbent 
for  Becquerel's  rays  than  for  Rontgen's  rays.  Becquerel's 
rays  will  discharge  an  electroscope  through  metal  screens, 
such  as  platinum  or  copper  1*4  mm.  thick,  which  would 
arrest  Rontgen's  rays.  Copper  and  aluminium  screens  are 
almost  equally  transparent.  Platinum  is  a  little  more 
absorbent.  The  absorption  of  metallic  screens  of  copper 
and  aluminium  together,  or  of  platinum  and  aluminium 
together,  is  less  than  the  sum  of  the  absorptions  by  the 
same  screens  separately,  thus  proving  that  the  Becquerel 
rays  are  not  homogeneous. 

Becquerel  proved  reflexion  by  laying  on  a  dry-plate  a 
lamella  of  uranico-potassium  sulphate  half-covered  with  a 
polished  steel  plate,  face  downwards.  The  covered  part 
was  more  strongly  affected  after  an  exposure  of  55 
hours.  He  also  demonstrated  reflexion  by  using  a  hemi- 
spherical tin  mirror.  Refraction  was  demonstrated  by  using 
a  thin  prism  of  crown  glass,  near  the  refracting  edge  of 
which  was  placed  a  tube,  i  mm.  in  diameter,  containing 
crystals  of  nitrate  of  uranyl.  After  three  days'  exposure  he 
found  the  light  refracted  towards  the  base  of  the  prism. 
He  proved  these  rays  to  be  capable  of  polarisation,  since 
the  photographic  shadow  through  two  plates  of  tourmaline 
was  stronger  when  their  axes  were  crossed  than  when  they 
were  parallel. 

Metallic  uranium  surpasses  all  other   materials  in  the 


APP.         OTHER  KINDS   OF   INVISIBLE   LIGHT          281 

freedom  with  which  it  gives  off  these  rays.  It  continues  to 
emit  them  for  months  in  complete  darkness  ;  the  source 
whence  this  supply  of  energy  is  derived  being  at  present 
unknown. 

The  author  made  some  experiments  to  compare  the 
penetrating  power  of  uranium  rays  with  that  of  Rontgen's 
rays.  Using  a  layer  of  uranium  nitrate,  he  found  carbon 
and  copal  to  be  as  opaque  as  rock  salt ;  whereas  with 
Rontgen's  rays  those  substances  are  much  more  transparent 
than  rock  salt. 

There  appears  to  be  no  doubt  that  the  uranium  rays  are 
a  species  of  extreme  ultra-violet  light,  having  a  wave-length 
certainly  less  than  10  micro-centimetres,  and  a  frequency 
certainly  greater  than  3000  billions  per  second. 

Quite  recently  Dr.  W.  J.  Russell  has  found  a  similar 
action  to  be  produced  by  newly  cleaned  metallic  zinc,  and 
by  some  other  materials,  including  some  kinds  of  wood  and 
paper. 

Phosphorus  Light. — The  author  has  examined  the  pene- 
trative effect  of  some  other  kinds  of  light.  The  pale  light 
emitted  by  phosphorus  when  oxidising  in  moist  air  is 
accompanied  by  some  invisible  rays  which  will  penetrate 
through  black  paper  or  celluloid,  but  will  not  pass  through 
aluminium.  So  will  some  invisible  rays  that  are  emitted  by 
the  flame  of  bisulphide  of  carbon. 

Light  of  Glow-worms  and  Fireflies. — Dr.  Dawson 
Turner  has  found  that  the  light  emitted  by  glow-worms 
contains  photographic  rays  which  will  pass  through  alu- 
minium. 

In  Japan,  Dr.  Muraoka  has  examined  the  rays  emitted 
by  a  firefly  ("  Johannis-kafer  ").  He  found  that  they  emitted 
rays  which,  after  filtration  through  card  or  through  copper 
plates,  would  act  photographically.  These  rays  can  be 
reflected,  and  probably  refracted  and  polarised.  He  used 
about  1000  fireflies  shut  up  in  a  shallow  box  over  the 
screened  photographic  plate. 

Wiedemantfs  Rays. — Professor  E.  Wiedemann  in  1895 
described  some  rays  (named  by  him  Discharge-rays,  or 
Entladungsstrahleti]  which  are  produced  in  vacuum-tubes 


282  LIGHT  LECT.  vi 

by  the  influence  of  a  rapidly-alternating  electric  discharge. 
They  have  the  property  of  exciting  in  certain  chemically 
prepared  substances,  notably  in  calcium  sulphate  containing 
a  small  percentage  of  manganese  sulphate,  the  power  of 
thermo-luminescence.  In  other  words,  the  substance  after 
exposure  to  these  rays  will  emit  light  when  subsequently 
warmed.  They  are  emitted  at  lower  degrees  of  rarefaction 
than  are  necessary  for  producing  the  kathode  rays.  They 
are  emitted  from  all  parts  of  the  path  of  the  spark-discharge, 
but  more  strongly  near  the  kathode.  They  are  propagated 
in  straight  lines,  but  no  reflexion  of  them  by  solid  bodies 
has  yet  been  observed.  They  are  readily  absorbed  by 
certain  gases,  oxygen  and  carbonic  dioxide,  but  their 
production  is  promoted  by  hydrogen  and  nitrogen.  Those 
produced  in  hydrogen  are  partially  transmitted  by  quartz 
and  fluor-spar.  They  are  apparently  not  present  in  the 
glow  discharge.  In  vacuo  these  rays  are  produced  by  all 
parts  of  the  discharge.  Under  the  influence  of  electric 
oscillations  they  are  emitted,  even  in  some  cases  at  half  an 
atmosphere  of  pressure,  at  the  boundary  of  the  rarefied  gas 
and  the  glass  wall,  even  before  any  visible  light  is  seen. 
No  deviation  of  them  by  the  magnet  has  yet  been  observ- 
able. Those  produced  at  relatively  great  pressures  have  in 
general  the  power  of  penetrating  bodies  according  to  the 
inverse  ratio  of  their  densities. 

New  kinds  of  Kathode  Rays. — The  author  has  recently 
found  two  new  kinds  of  kathode  rays.  One  of  these, 
termed  parakathodic  rays,  is  produced  when  ordinary 
kathode  rays  strike  upon  an  anti-kathode,  as  in  the  "  focus  " 
tubes.  If  the  vacuum  is  low,  there  are  emitted  from  the  anti- 
kathode,  in  nearly  equal  intensity  in  all  directions,  some 
rays  that  closely  resemble  ordinary  kathode  rays.  They  can 
be  deflected  electrostatically  and  magnetically,  and  can  cast 
shadows  of  objects  on  the  glass  walls.  If  the  vacuum  is 
high  enough  for  the  production  of  Rontgen's  rays,  some 
parakathodic  rays  are  also  produced  at  the  same  time. 
They  cause  the  glass  bulb  to  fluoresce  over  an  obliquely 
limited  region  as  in  Fig.  142,  p.  265. 

The  other  kind,  termed  diakathodic  rays,  is  produced 


APP.         OTHER    KINDS   OF   INVISIBLE   LIGHT          283 

by  directing  the  ordinary  kathode  rays  full  upon  a  piece  of 
wire-gauze,  or  upon  a  spiral  of  wire,  which  is  itself  negatively 
electrified.  The  ordinary  kathode  rays  refuse  to  pass  through 
the  meshes  of  the  gauze,  but  instead  there  passes  through 
abeam  of  bluish  rays,  which  differ  from  kathode  rays  in  that 
they  are  not  directly  affected  by  a  magnet.  These  diakathodic 
rays  can  also  produce  fluorescence  of  the  glass  where  they 
meet  the  walls  of  the  tube,  and  can  cast  shadows  of  inter- 
vening objects  ;  but  the  fluorescence  is  of  a  different  kind, 
for  ordinary  soda  glass  gives  a  dark  orange  fluorescence 
instead  of  its  usual  golden  green  tint.  This  orange  fluor- 
escence when  examined  by  the  spectroscope  shows  the 
D-lines  characteristic  of  sodium. 

Goldstein's  Rays. — Herr  Goldstein  has  also  described 
some  rays  apparently  closely  akin  to  those  just  mentioned. 
If  a  perforated  disk  is  used  as  a  kathode  there  are  produced 
some  blue  rays  which  stream  back  behind  the  kathode 
opposite  the  apertures. 


INDEX 


ABNEY,   Captain  W.    de  W.,   on 

emulsion  films,  166 
on  colour  vision,  183  footnote 
on  minimum  visible  luminosity, 

211 
Absorption    of  light    by  coloured 

media,  88 

and  anomalous  dispersion,  104 
by  black  surfaces,  201 
of  Rontgen  rays,  243 
Acetylene  lamp,  20 
Actinic  waves,  162,  182 
Ahrens's  polariser,  123 
Air-pump,  the  mercurial,  247,  251 
thermometer,    experiment  with, 

202 

Aladdin,  lamp  of,  180 
Alexandrite,  colours  of,  86 
Aluminium,     transparency    of,    to 

Rontgen  rays,  243 
leaf,    use    of,    in    electroscopes, 

269 

Amethyst,  optical  properties  of,  134 
Ampere's  construction,  69 
Amplitude  of  wave-motion,  9 
Analyser,  117 
Analysis    of    light    by    prism    or 

grating,  79,  86 
Animatograph,  97 
Anomalous  refraction,  100 
Anschiitz's  moving  pictures,  97 
Antikathode,  the,  265 
Antipyrin,    optical    properties   of, 
150 


Arc-lamp,  emission  of  light  by,  1 1 1 

whiteness  of  light  of,  211 
Artificial  rainbow,  80 
Ayrton  and  Perry,  on  ratio  of  the 
electric  units,  233 

BECQUEREL,  Professor  Henri,  his 

discovery  of  radiations  from 

uranium  salts,  272,  278 
Becquerel's  rays,   diselectrification 

by,  279 

frequency  of,  281 
Benetto's  method  of  photography 

in  colour,  184  footnote 
Benham's  colour-top,  96 
Bidwell,    Shelford,    on    fatigue    of 

retina,  96 

strange  colour-effects,  99 
Black,  is  mere  absence  of  light,  72 
cross,  in  polariscope,  134,  152 
surfaces  absorb  waves  and  grow 

warm,  202 
surfaces     radiate     better     than 

bright,  203 
Blue  and  yellow  make  white,  89, 

146,  188 

Blue  of  the  sky,  theory  of,  233 
Bologna  stone,  177 
Bolometer,  use  of,  197 
Boltzmann,  Professor  Ludwig  von, 

on  electro-optics,  234 
Bose,    Professor  J.    Chunder,    on 

polarisation  of  electric  waves, 

227 


286 


LIGHT 


Bose,  Professor  J.  Chunder,  his 
apparatus  for  optical  study  of 
electric  waves,  226 

Boyle,  Hon.  Robert,  on  phosphor- 
escence of  diamonds,  177 

Bright  field,  118,  212. 

Brodhun  and  Lummer  Photo- 
meter, 19 

Brightness  of  lights,  14 

Burning-glass,  37 
mirror,  206 

CALC-SPAR  :  see  Iceland  spar 
Calorific  waves,  162,  193,  197 
Campbell  -  Swinton,     A.     A. ,    his 
photographs  by  Rontgen  rays, 
270 

Canton's  phosphorus,  175 
Casciarolo  of  Bologna,  177 
Chemical  effects  of  waves,  163,  166 
Chemi-luminescence,  175,  176 
Christiansen    on    anomalous    dis- 
persion, 100 

Cold,  apparent  radiation  of,  205 
Colour  and  wave-length,  71,  72 

sensations,  primary,  183 

top,  Benham's,  97 

vision,  183 
Coloured  stuffs  viewed  in  coloured 

light,  82 

Colours  that  are  not  in  the  spec- 
trum, 87 

complementary,  91,  in,  188 

of  polarised  light,  136 

of  thin  plates,  137 

of  soap  bubbles,  138 

supplementary,  149 
Combination  of  colours,  84,  8=;,  142 

to  produce  white  light,  83,  146 
Complementary  colours,  91,  188 

tints,  91,  93,  in,  188 

in  bright  and  dark  field,  136 

in  double-image  prism,  149 
Concave  lens,  diverges  light,  41 

mirror,  reflexion  by,  27 
Contrast  tints,  92,  93,  99 
Convergence  of  light  to  focus  by 
reflexion,  26 


Convergence  of  light  to  focus  by 

refraction,  35,  37,  41 
Convex  lens,  converges  the  light,  40 

mirror,  reflexion  by,  24,  28 
Cornea  of  eye,  44 
Corpuscular  theory  of  light,  230 
Critical  angle,  39 
Crookes,    Professor    William,    his 

radiometer,  199,  213 
tube  used  by  Rontgen,  239,  241 
improvements  in  vacuum  pump, 

250 

radiometer,  252 
on   repulsion  due   to  radiation, 

252 
on  properties  of  kathode  rays, 

253 
views  on  radiant  matter,    253, 

258 

tube  with  shadow  of  cross,  255 
invented  focus  tube,  256 
Cryptoscope,  268 
Crystalline  lens  of  the  eye,  44 
Crystallo-luminescence,  175 
Crystals,  elasticity  of,  129 
Curvature,    printed    on   the  wave- 
front,  24 
imprinted    by   lens    or    curved 

mirror,  43,  56,  67 
expansion  of,  61 
Cylindrical  lens,  49,  83 

DARK-FIELD,  119,  120 

for  polarised  heat-waves,  212 

Davy,  Sir  Humphry,  on  reflexion 
of  heat,  206 

Delezenne's  polariser,  123 

DenSer  medium,  35 

Density  and  refractivity,  49 

Detection  of  false  gems  by  polar- 
ised light,  133 
by  Rontgen  rays,  272 

Detectors  of  electric  waves,  217, 
223,  226 

Dewar,  Professor  James,  on  phos- 
phorescence of  bodies  cooled 
in  liquid  air,  179 

Diakathodic  rays,  282 


INDEX 


287 


Diamond  does  not  polarise  light, 

i34 

phosphorescence  of,  177,  178 
transparency    of,     to     Rontgen 

rays,  270 

Dielectrics,  104,  232 
Difference  of  phase,  136 
Diffraction-grating,  31,  77,  7$ 

spectrum,  78 
Diffuse  reflexion,  30 
Dioptrie,  definition  of,  59 
Direction     of     the    vibrations    in 

polarised  light,  233 
Diselectrification    by    ultra  -  violet 

light,  181 

by  Rontgen  rays,  268 
by  uranium  rays,  288 
Dispersion  of  light  by  prism,  74 
anomalous,  100 
and  frequency,  158 
Divergence  of  light  from  focus  by 

reflexion,  24 
by  refraction,  41 
Divergivity,  62 
Double-image  prism,  125 
Double  refraction,  120 

refraction,     Lord    Rayleigh    on 
theory  of,  233 

EBONITE,     transparency     of,     for 

heat  waves,  213 
optical  and  dielectric  properties 

of,  233 

P^ffluvio-luminescence,  175 
Elasticity  in  crystals,  129  footnote 

axes  of,   131 

Elastic-solid  theory  of  light,  156 
Electric  oscillations,  219 
waves,  214 
waves,    prediction    of,    by  Clerk 

Maxwell,  229 
sparks  in  vacuo,  luminosity  due 

to,  247 

sparks,  oscillatory,  218 
Electricity    discharged    by    ultra- 
violet light,   181 
by  Rontgen  rays,  268 
by  uranium  rays,  288 


Electro-luminescence,  175 
Electromagnetic  theory  of  anomal- 

ous dispersion,  102 
theory  of  light,  232 
Electroscope,  used  for  photo-elec- 

tric experiments,  181 
as    detector    of    electric    waves, 

223 

with  aluminium  leaves,  181,  269 
use  of,  by  Lenard,  259 
use    in    observing  diselectrifica- 

tion,  269 
Emission  of  light  at  different  tem- 

peratures, 174 
Emulsion    films    in    photography, 

166 

Ether,  the,  108,  230 
Eye,  the,  sensitiveness  of,  14 
as  optical  instrument,  43 
images  are  inverted  in,  45 
unable    to    detect    polarisation, 

in 
of   codfish,    in    polarised    light, 


FAIRY  FOUNTAIN,  39 

Faraday,    Professor   Michael,   first 

experiments  in  electro-optics, 

229,  230 
his    electromagnetic    theory    of 

light,  231 
Fatigue,  effects  of,  14  footnote,  93, 

c6 
P'echner's  law  of  magnitude  of  sen- 

sation,  1  4  footnote 
Filter  -  screens    for    invisible   light, 

164,  213 

Fireflies,  luminescence  of,  176 
rays   emitted   by,   pass   through 

copper,  281 
FitzGerald,   Professor  George   F., 

on  electromagnetic  theory  of 

reflexion  and  refraction,  233 
on  starting  waves  in  the  ether, 

234 
Fizeau,    on   number   of  waves   in 

train,  112 
Flames,  radiation  of  heat  by,  203 


288 


LIGHT 


Fleming,  Professor  J.  A. ,  magnetic 

action  on  kathode  rays,  255 
Fluorescence,  phenomenon  of,  169 
experiments  in,  170 
table  of,  175 
Fluorescent  screens,  268 

use  of,  in  ultra-violet  light,  172 
Fluorescope,  268 

Fluor-spar,  transparency  to  ultra- 
violet light,  164,  1 66 
photo  -  luminescence      (fluores- 
cence) of,  169 

ther mo-luminescence  of,  180 
Focus,  real,  26,  37 
virtual,  26 

tubes,  Crookes's,  256 
,,       the  author's,  265 

Jackson's,  266 
,,        Bohm's,  267 
Formulae  for  refraction,  61 
for  lenses,  65,  67 
for  reflexion,  67 
Foucault's  modification  of  Nicol's 

prism,  121 
Frequencies    and    wave  -  lengths, 

table  of,  190 
Frequency  of  sound  waves,  106 

of  different  colours,  190 
Fresnel's  theory  of  light,  157 
views   as  to  direction  of  vibra- 
tions, 233 

GALITZINE,  Prince,  on  alleged 
polarisation  of  Rontgen  rays, 
261 

Gases,  optical  and  dialectric  pro- 
perties of,  233 
glow  in  vacuum  tubes,  248 

Geissler's  tubes,  247 

Gems,  optical  properties  of,  86, 
119,  120,  133,  134,  178,  254 

Geometrical  optics,  methods  of,  55 

Germany,  research  in,  263 

Gifford,  J.  W. ,  his  photographs  by 
kontgen  rays,  270 

Gladstone,  Dr.  J.  Hall,  on  photo- 
graphing the  invisible,  168 

Glass,  velocity  of  light  in,  35 


Glass,     polarising     properties     of 

unannealed,  151 
strain  in,  152 
absorption  of  ultra-violet    light 

by,  164 
opacity  of,  to  ultra-violet  light, 

166,  174 

opacity  of,  to  heat-waves,  198 
opacity    of,    to    Rontgen    rays, 

271,  272 
Glazebrook's    Report    on    Optical 

Theories,  158 
Glow-worms,  luminescence  of,  176, 

178 
Glow-worm  light  will  pass  through 

aluminium,  281 

Goethe,  his  theory  of  colours,  76 
Goldstein,    Dr.    Eugen,    on    rays 

behind  the  kathode,  283 
Granite,  in  polarised  light,  134 
Grating,  diffraction  produced  by, 

3i 

Green  cannot  be  made  by  mixing 
pure  yellow  and  blue,  89 

H AIDINGER'S  brushes,  1 1  \footnote 
Hartnack's  modification  of  Nicol's 

prism,  121 
Hauksbee,     Francis,     on     electric 

luminosity,  246 
Heat-indicating  point,  206 
waves  reflected  to  focus,  206 
shadows,  209 
spectrum,  162,  197 
Heating  effect  of  waves,  162 

by  absorption  of  waves,  201 
Heaviside,  Oliver,  on  propagation 

of  energy,  233 
Hefner's  standard  lamp,  20 
Helmholtz,     Hermann     von,     on 

anomalous  refraction,  102 

on  electromagnetic  theory,  234 

Herschel,  Sir  J.  W.  F. ,  on  plane 

of  polarisation,  158 
Sir  William,  on  heat  spectrum, 

200 

Hertz,  Professor  Heinrich,  on  dis- 
electrification,  181 


INDEX 


289 


Hertz,     Professor    Heinrich,    dis- 
covery of  electric  waves,  214 
on  oscillatory  sparks,  214 
his  oscillators,  215,  221 
on     reflexion     of    waves,     217, 

220 

effect  of  his  discoveries,  234 
waves,   model    illustrating    pro- 
pagation of,  237 
on  transparency  of  metal  films 

to  kathode  rays,  258 
Hittorf,  Professor  W. ,  on  kathode 

phenomena,  252 

Hopkinson,  Professor  John,  on 
optical  and  electric  proper- 
ties, 233 

Horn,  optical  properties  of,  150 
Home's  luminescent  stuff,  178 
Huy gens' s  principle  of  wave  pro- 
pagation, 9 
construction,  69 
Hyperphosphorescence,  278 

ICE,  apparent  radiation  of  cold 
by,  205 

optical  and  dielectric  properties 

of,  233 

Iceland  Spar,  120,  129,  174,  198 
Illumination  of  a  surface,  15 
Image  of  a  luminous  point,  22 

in  mirror,  position  of,  23 
Images,  formation  of,  28 

inverted,  30 

in  eye  are  inverted,  45,  47 
Incandescence,  the  process  of,  210 
Infra-red  waves,  192 
Interference  of  waves,  10 

of  light  produces  colours,  141 
Internal  reflexion,  39 
Inverse  squares,  law  of,  16 
Invisible     spectrum,     ultra  -  violet 
part,  1 60 

infra-red  part,  192 
Invisible,  the  photography  of  the, 

168 

Iodine  vapour,  anomalous  refrac- 
tion of,  100 
Irrationality  of  dispersion,  78 


Ives's  method  of  registering  colour 

by  photography,  185 
photochromoscope,  187 

JACKSON,    Professor  Herbert,   his 

focus  tube,  266 
Japanese  mirrors,  59 
Jelly,  vibrations  transmitted  by,  1 10 

KALEIDOSCOPE,  principle  of,  33 
Kathode  rays.  253 

focusing  of,  256 

name  inappropriate,  258 

new  varieties  of,  266 
Kathode,  phenomena  at,  249 

shadows,  252,  254 

,,         magnetic  deflexion  of, 

255.  257 

streams,  253,  256 
Kathodo-luminescence,  175 
Kearton,  J.  W. ,  his  magic  mirrors, 

S3 

Kelvin,  Lord,  theory  of  the  ether, 

234 

Kerr,  Dr.  John,  magneto  -  optic 
discoveries,  234 

Kromskop,  187 

Kundt,  August,  on  anomalous  re- 
fraction, 101,  103 

LAMP,    arc-,    images    of   carbons 

in,  29 

Hefner's  standard,  20 
monochromatic,  82 
Langley,     Professor     S.     P.,     on 

longest  waves,  190,  197 
his  bolometer,  197 
Law  of  Fechner,  14  footnote 

of  inverse  squares,  16 
Le  Roux  on  anomalous  dispersion, 

100 
Lenard,     Professor     Philipp,     his 

researches,  258 
diselectrifying  effect   of  kathode 

rays,  259 

Length  of  wave  :  see  wave-length 
Lens,  crystalline, principle  of,  36,40 
of  eye,  44 


290 


LIGHT 


Lens,  cylindrical,  49 

measurer,  59 

Light,  velocity  of,  2,  129,  156 
Lippmann,   Professor  Gabriel,  on 

photography  in  colour,  184 
Lodge,    Professor    Oliver   Joseph, 

his  oscillators,  222 
his  detector,  223 
his  apparatus  for  optical   study 

of  electric  waves,  224 
on  electric  oscillations,  234 
illustrations  of  Maxwell's  theory, 

235 

London,  University  of,  contrasted 
with  that  of  Wurzburg,  262 

Longest  waves  of  infra-red,  190 

Luminescence,  174 

Luminescent  screen  in  ultra-violet 

light,  i 

used  by  Rontgen,  239,  241 
best  kind  of,  268 

Luminous  paint,  177 

Lummer,    Prof.   Otto,   his   photo- 
meter, 20  footnote 

MAcCuLLAGH's   theory  of  light, 

157 

views  as  to   direction  of  vibra- 
tions, 233 
Magenta,  absorption  spectrum  of, 

87,  104 
anomalous   refraction    of,    100, 

104 

Magneto-optic  discoveries,  230,  234 
Maxwell,   Professor   James  Clerk, 

on  colour-vision,  183 
predicted  electric  waves,  214 
electromagnetic  theory  of  light, 

229 
on    Faraday's     electromagnetic 

theory,  231 
Mercurial  air-pump,  247,  251 

phosphorus,  246 
Mica,   optical   properties   of,   137, 

146 
Michelson,  on  number  of  waves  in 

train,  112 
Miller's  limit  of  shortest  waves,  191 


Mirage,  experiment  illustrating,  48 
Mirror,    magic,    reflexion   of  light 

by,  21 

of  Japan,  50 
English,  53 

Mirrors,  concave  and  convex,  24 
paraboloidal  for  reflecting  heat- 
waves, 206 

parabolic,  used  by' Hertz,  220 
Model  illustrating  Stokes's  theory 

of  Rontgen  rays,  275 
illustrating  propagation  of  Hertz- 
wave,  237 
Models  of  wave -motions,  8,  109, 

115,  124,  130 

Molecular  bombardment,  253 
Monochromatic  lamp,  82 
Monoyer  on  definition  of  dioptric, 

59 
Morton,      President     Henry,     his 

fluorescent  dyes,  171 
Mother-of-pearl,  colours  of,  79 
Moving  pictures,  97 
Muraoka,    Dr.,   on   fire -fly  light, 

281 
Muybridge,    on      movements      of 

animals,  97 

NEW  kinds  of  kathode  rays,  282 
Newton's  colour-whirler,  34 

theory  of  nature  of  white  light, 

77-  83,  86 
tints,  137 
rings,  138 
table  of,  140 
explanation  of,  142 
Nichols,  Edward  Fox,  on  anomal- 
ous refraction,  104 
and  Rubens,  on  longest  waves, 

197 
Nicol,     William,     his     polarising 

prism,  121 

prism,  modern  varieties  of,  121 
Nodal   points   in   reflected  waves, 
217 

OPACITY  and  electric  conductivity, 
relation  between,  232,  234 


INDEX 


291 


Opacity  and  electric  conductivity, 

of  electric  conductors,  232 
Opacity  of  glass  to  invisible  light, 

166,  174,  198,  271 
of  metals  to  Rontgen  rays,  243 
Optical   circle,    for  demonstrating 

refraction,  38 
illusions,  92,  94,  96,  99 
rotator,  123 

Orders  of  Newton's  tints,  139 
Ordinary  and  extraordinary  waves, 

I25 

Orthochromatic  photography,  183 
Oscillating  sparks,  214,  218 
Oscillator,  Hertz's,  215,  221 

PAINT,  luminous,  177,  178 

heat-indicating,  208 
Parabolic  mirrors,  Hertz's,  220 
Paraboloidal  mirrors  for  reflecting 

heat,  206 

Paraffin  oil,  fluorescence  of,  169 
Paschen's     longest     waves,      190, 

197 

Pentadecylparatolylketone,  241 
Permanganate   of  potash,  absorp- 
tion by,  88 
Perrin,     Jean,     on     refraction    of 

Rontgen  rays,  261 
Persistence  of  vision,  93,  97 
Petroleum,  fluorescence  of,  169 
Phase,  difference  of,  136 
Phenakite ,    kathodo  -  luminescence 

of,  254 

Phosphorescence,  175 
Phosphorus,  luminescence  of,  176 
Canton's  artificial,  177 
the  mercurial,  246 
nature  of  rays  omitted  by,  281 
Photochemical  effects  of  light,  163, 

166 

Photochromoscope,  187 
Photoelectric  effects  of  light,  181, 

268,  279 

Photographic  waves,  162,  182 
spectrum,  166 

registration  of  Rontgen  shadows, 
245 


Photography  of  the  invisible,  168 
.   in  natural  colours,  183,  184 

the  "  new,"  245 
Photo-luminescence,  175,  176 
Photometer,  14 

Thompson  and  Starling's,  18 

Trotter's,  18 

Brodhun    and     Lummer's,     19 
footnote 

Bunsen's,  19  footnote 

Joly's,  19 

Pigments  darken  the  light,  83 
Pink  not  a  spectrum  colour,  87 
Plane -waves,  7 

Platinocyanides,  their  optical  pro- 
perties, 172 
Polarisation,  105 

plane  of,  158 

of  electric  waves,  225,  227 

of   Rontgen  rays,  attempts   at 
261 

of  BecquereTs  rays,  280 
Polariscope,  simple,  153 
Polarisers,  different  kinds  of,  113 
Positive    curvature,    definition    of, 

59 

lens,  definition  of,  59 
Poynting,     Professor    J.     H.,   on 
ripple -tanks,  6 

on  energy  paths,  233 
Primary  colour  sensations,  86,  89, 
91.  183 

tints,  86,  91 
Prism,  refracting,  74 

direct-vision,  80 

Foucault's,  121 

Hartnack's,  121 

Nicol's,  121 

double-image,  125 
Prismatic  spectrum,  74 

irrationality  of,  78 
Propagation  of  waves,  9 

of  light  in  glass,  35 

of  waves  longitudinally,  106 

of  waves  transversely,  107,  237 
Purple    not    a    spectrum    colour, 
87 

analysis  of,  87 


292 


LIGHT 


QUARTER-WAVE  plate,  148 
use  of,  123  footnote,  148 

Quartz,  anomalous  dispersion  of, 

104 
rotatory   optical   properties   of, 

154 
transparency  of,  to   ultra-violet 

light,  164 
lenses  and  prisms,  use  of,  164, 

170,  174 

tribo-luminescence  of,  181 
transparency  of,  to  heat-waves, 

198 
Quinine,   fluorescent  property  of, 

168,  171,  172 

RADIANT  heat,  193 

matter,  253,  258 
Radiation,  193  footnote 
Radiometer,  Crookes's,  199,  213 
Rainbow  due  to  refraction,  73 

artificial,  80 
Ray-filters  for  infra-red  light,  213 

for  ultra-violet  light,  164 
Rayleigh,    Lord,    on   shadows    of 
sounds,  5 

on  anomalous  refraction,  101 

Theory  of  Sound,  107 

on    electromagnetic    theory   of 
light,  233 

on  blue  of  the  sky,  233 

on  double-refraction,  233 
Rays,  non-existence  of,  12 ".footnote 

kathode,  use  of  term,  257 
Real  focus,  26 
Reflexion  by  plane  mirror,  21 

by  convex  mirror,  24 

by  concave  mirror,  26,  27 

irregular  or  diffuse,  30 

by  multiple  mirrors,  33 

of  heat-waves,  206,  209 

alleged,  of  Rontgen  rays,  260 
Refraction  of  light,  35 

anomalous,  100 

double,  1 20 

feeble,  of  Rontgen  rays,  261 
Resolution  of  vibrations,  126 
Resonator,  Hertz's,  216 


Retina  of  the  eye,  45 

fatigue  of,  93,  96,  99 
Righi,  Professor  Augusto,  his  os- 
cillators, 221 
his  apparatus  for  optical  study 

of  electric  waves,  225 
Ripples,  on  water,  6 

convergence  and  divergence  of, 

12 

Ripple-tank,  6 
Rock-salt  lenses  and  prisms,  use 

of,  194 
transparency  to  ultra-violet  light, 

166 
transparency  to  infra-red  light, 

198 
Rontgen,  Professor  Wilhelm  Kon- 

rad,  262 

account  of  his  discovery,  238 
his  form  of  tube,  260 
his  theory  of  the  rays,  273 
Rontgen  rays,  properties  of,  240 
penetrative  power  of,  243 
not  deflected  by  magnet,  260 
are  not  kathode  rays,  260 
are   not    ordinary   ultra  -  violet 

light,  260 

are  not  reflected,  260 
point  of  origin  of,  264 
curious  lateral  emission  of,  265 
shadows  of  bones  made  by,  269 
speculations  as  to  nature  of,  273 
Rosaniline,  absorption  spectrum  of, 

87,  104 
anomalous   refraction    of,    100, 

104 
Rotation     of    polarised    light   by 

quartz  and  sugar,  154 
in  magnetic  field,  230,  234 
by  reflexion  at  magnet  pole,  234 
Rotator,  optical,  123 
Rowland's  ruling  machine,  77 
Rubens' s    and     Nichol's     longest 

waves,  190 

Rubies,  real  and  sham,  133 
glow  in  kathode  stream,  254 
transparency    of,    to     Rontgen 
rays,  272 


INDEX 


293 


Rumford,  Count,  on  radiation  of 

heat,  207 
Rupert's  drops,  optical  properties 

of,  151 
Russell,  Dr.  W.  J.,  on  new  kinds 

of  invisible  rays,  281 

SALICINE,    optical    properties   of, 

i35 
Schumann's  limit  of  shortest  waves, 

191 
Selenite,  crystal  films  of,  133,  136, 

145,  148 

Sensitiveness  of  eye,  14 
Shadows,  light  penetrates  into,  4, 

12 

of  sounds,  4 

of  heat,  209 

cast  by  Rontgen  rays,  242 

kathodic,  in  Crookes's  tubes,  255 
Shortest  waves  in  ultra-violet,  191 
Silk,  shot,  reflexion  by,  31 
Smoothness,  optical  definition  of, 

21 

Soap-bubbles,  colours  of,  138 
Sound-waves,  size  of,  3 
Spar,  calc  :  see  spar,  Iceland 
Spar,  Iceland,  120,  129,  174 
Spectrum  analysis,  vii,  87 

of  colours,  74 

produced  by  prism,  75,  100 

produced  by  diffraction-grating, 

77 

visible  part  of,  161 

invisible  parts  of,  161 

the  photographic,  166 

the  long,  173 

Speed  of  light  :  see  Velocity 
Spherometer,  58 
Sprengel's  vacuum-pump,  250 

Crookes's  improvements  in,  250 
Standard  candle,  14,  20 

lamp,  20 

Stationary  waves,  184,  217 
Stokes,   Sir    George    Gabriel,    his 
discoveries    in     fluorescence, 
169,  172 

shortest  waves  observed  by,  191 


Stokes,  Sir  George  Gabriel,  his 
theory  of  Rontgen  rays, 
273 

Strain     in     imperfectly     annealed 

glass,  151 
in  compressed  glass,  152 

Strobic  circles,  96 

Subjective  colours,  92 

Sugar,  luminescence  of,  108 
rotatory  properties  of,  155 

Supplementary  tints,  149 

TALBOT  on  anomalous  refraction, 
100 

Temperature  in  relation  to  emis- 
sion of  light,  174,  211 

Tints  of  spectrum,  72 

complementary,    91,     93,     in, 

136,  149 

Newton's,  137,  142 
supplementary,  149 

Thaumatrope,  94 

Thermo-luminescence,  175,  180 

Thermometer,  to  explore  infra-red 

spectrum,  193,  200 
air,  experiment  with,  202 

Thermopile,  use  of,  194,  196,  203, 
207,  212 

Thomson,  Professor  Joseph  John, 
on  ratio  of  units,  233 

Three  -  colour  method  of  photo- 
graphy, 183 

Total  internal  reflexion,  39 

Tourmaline,  optical  properties  of, 

119,  120 
opacity  and  conductivity  of,  234 

Trains  of  waves,  112,  219,  222,  273 

Transition-tint,  139 

Transparency  of  flesh,  leather,  and 
paper,  243 

Transverse  waves,  108 

Tribo-luminescence,  175,  180 

Trichromic  theory  of  colour- vision, 
183 

Turner,  Dr.  Dawson,  on  glow- 
worm light,  281 

Tyndall,  Professor  John,  his  use 
of  colour  disks,  92 


294 


LIGHT 


Tyndall,  Professor  John,  experi- 
ments on  reflexion  of  heat- 
waves, 209 

on  sound,  107 

lectures  on  light,  171 

his  wave-filter  for  infra-red,  213 

ULTRA-VIOLET  light,  160,  162 
chemical  effects  of,  167 
reflexion,  etc.,  of,  174 
light,  diselectrification  by,  181 
Universities  of  London  and  Wiirz- 

burg,  262 

Uranium  glass,  fluorescence  of,  169 
nitrate,    tribo-luminescence    of, 

180 
Uranium,  great  density  and  opacity 

of,  244 
rays,  278 

VACUUM  -  PUMP,    the    mercurial, 

247-  251 
Velocity  of  light  in  air,  2,  33 

in  water,  33 

in  glass,  33 

of  heat-waves,  212 

of  propagation   of   electric  dis- 
turbances, 232 

constant,  definition  of,  60 
Virtual  focus,  26,  27 

WAVE-LENGTH,  4,  7,  72 
motion  models,  8,  124,  236 
front,  motion  of,  9 
table  of,  72,  190 
filters  for  infra-red  waves,  213 


Wave-length  of  electric  waves, 
214 

theory  of  light,  230 
Waves,  travelling  of,  7,  106,  236 

propagation    of,    9,     106,    157, 

233 
trains  of,  112,  219,  222,  273 

Weber,  Professor  Wilhelm,  on 
ratio  of  electric  units,  232 

Webster,  Rt.  Hon.  Sir  Richard, 
his  hand,  270 

Wheatstone,  _  Sir  Charles,  on 
velocity  of  electric  disturb- 
ances, 232 

Wheel  of  life,  97 

White  light,  analysed,  79,  149 
synthesis  of,  83,  84,  90 

Whiteness,  no  standard  of,  known, 
211 

Wiedemann,  Professor  Eilhard,  on 

luminescence,  174,  180 
his  "discharge-rays,"  276,  281 

Winkelmann,  Professor  A.,  on  re- 
fraction of  Rontgen  rays,  261 
footnote 

Wiirzburg,  University  of,  263 

X-LUMINESCENCE,    175,   260 

X-rays  :  see  Rontgen  rays 

YELLOW  not  a  primary  colour- 
sensation,  89 

Young,  Dr.  Thomas,  on  colour- 
sensation,  183 

ZOETROPE,  97 


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